The middle camps in smite are a highly contested and important aspect of winning your conquest matches. They will spawn in at 3 minutes exactly on either side of middle lane;

The Location of Mid Harpies in Smite

The location of the buff camps is critical, you need to be able to get there just before 3 minutes, arrive at 3 minutes and the enemies may be able to nuke it down before you.
Hand of the gods is required to be up at this point if you're a god that purchases it. If you do not have it up then be prepared to engage on the enemy that's taking it, or have a level advantage.

Important Factors to Consider:

Level Advantage; Is the enemy a higher level than you? Can you duel them at this level? A level 4 to a level 5 difference is one of the most game changing differentials between levels, not having your ultimate against the opponents can be devastating. Alternatively, if you're a higher level you should pressure them more.

God Matchups; Is your god particularly good at taking harpies or dueling at your current level? It's okay to burn a low level ultimate to secure the middle camps. Certain Gods excel at the middle camps due to already having pushed out their laner and having a strong early game? Good examples of gods that are amazing at mid camps are Ymir, Athena, Agni, Poseidon, Ra, Thor and Bastet. All having insane power spikes at level 5 or easy to hit AOE CC. Agni in particular can devastate a team grouped up to take middle camps while also securing them himself.

Current State of HP, Mana and Items; You need to consider the following, if your HP is low, then you need to return to base in order to contest the middle camps adequately, even if you have been bullying out your opposing lane, have a level advantage and a gold advantage you need to watch your health, being below half health is a no go, and you must return to base in time for the camps.
It takes a no boots Ymir 30 seconds to get to the mid camps from spawn, so time your back accordingly, if you have any boots, take the time down to about 27 seconds(lower for higher tier boots), if you have a dash or movement speed buff the time can go down even further depending on the dash and cooldown of the dash.

Which Mid camp to go for?

As a general rule of thumb you should always contest the middle camp closest to your duo lane with your support, middle lane and jungler being available at this time. The Jungler should have backed atleast once in this time, while there is a option for the support and middle to back it isn't always the most effective, consider everything above, and if backing gives you a large enough advantage in health, mana and items to counter weigh the experience lost.

When to Rotate?

How to rotate for Middle camps (Color coding below)

As a Support (Blue Lines): Try and begin rotating around 2:45, or if you need to back to base then back to base within 2:20, so you have an entire 30 seconds to reach the middle camps. If you have a dash (Athena, Guan Yu) or a Movement speed buff (Sun Wukong) You can afford to rotate around 2:50 or back at 2:30, but be wary, because you won't have the alternate affects of the skills if you do so (Slow and damage or stun and damage)
Path 1: Standard, no returning to base
Path 2: Backing to base, taking this route

As a Mid Lane (Red Lines): Begin rotating at 2:54, it doesn't take long for you to arrive, but take the safest route, if you're pushed up at the enemy tower it may be worth rotating all the way back through your lane and into the jungle at your side in order to save any issues of being caught out by the enemy jungler, especially one that can deal great damage to a mage at level 5 (Thor,Loki,Bastet) If you need to back, do it at 2:25, as you will likely have more gold and larger boots than your support
Path 1: Standard route
Path 2: Pushed up, Safety route.

As a Jungler (Green Lines): Start the rotation based on how close you are to the middle camps, you need to arrive at 2:55 , try and base your jungle route around this also, backing to base right before the jungle camps are up as a jungler is highly recommended, as the power spike you gain with items is immense, especially if you have amassed enough gold to purchase Warrior Tabi/Ninja Tabi straight up (Or Boots of Magi for magical damage junglers)

As a Solo laner (Optional) (Yellow Lines) : This is very optional ,and essentially secures you the left camps if you decide to do so, you should do this on gods that are very good roamers and can afford to lose a wave , possibly in return for nothing . You won't/shouldn't be backing at 2:20 as a solo laner, either you have survived the first minute and a half with jungle presence on both sides, or you needed to back at that time, if you lack sustain, then you shouldn't be contesting the middle camps unless your health is high enough. Be sure to go for the alternate Mid Harpies
Path 1: Standard route
Path 2: Backing to base, taking this route

All of these paths can be reversed for the alternate team, seeing as the map is now symmetrical upon season 2 .

The Strength of Middle camps:

Comparing the Experience and gold gain from all camps, we can distinctly see why the middle camps are so strong;

Mid harpies: (140xp + 45g) x2 = 280xp + 90gold
Buff camps: 185xp + 65g + (22xp + 13g) x2 = 229xp + 91gold
Back harpies: 140xp + 45g + (22xp + 13g) x2 = 184xp + 71gold

With only one gold less than a buff camp, and 50 additional experience, the power of them is clear, they need to be contested, and the team that contests this camp in the smartest fashion is likely going to win the game, the advantage they provide is immense. The experience gained can catapult your lane into a lead that then snowballs into your favour heavily.

After the first middle camp: Keep a note of the time that the middle camps were killed, they will not spawn every 3 minutes exactly, but rather take 3 minutes to spawn after they were killed, this is why making sure you know when they were killed is crucial, you do not want to be there 20 seconds early, wasting time, but you also do not want to be there too late, already missed out on the camps, and potential time elsewhere.