More info is needed to fully solve your problem, such as your location (to know weather patterns), whether you use CO2, what types of lights, etc. I have experience with a room very similar to yours at about 18x18 running 9 lights of 600W HPS. To answer, I will assume each light you use is a 600W HPS, your outside air (ambient) temperature is <80 degrees. For the THC question, any heat stress will first cause the plant's growth to slow, which means its energy is not going into producing flowers/THC-- which is the biggest issue and is difficult to notice, but some signs are the curled edges of fan leaves. A plant sitting under a hot light will have a cola much hotter than the air temperature, so its likely your plants' tops are reaching 95+ if the air is 88 at plant height, which will damage trichomes/THC. (you can get a laser thermometer for under $20) Further damage will be seen in signs of wilting. If you are running that hot, your humidity may be rampant also, but with 3 incoming fans (CFM rating on fans is needed here to calculate) that should not be an issue. The question is what happens to the incoming air. If you have your room sealed in plastic and the only exiting air is through the hoods, I assume the intake portion of your hood vent is open to the air in the room. The rest of the air might be getting pushed out through cracks. If not then disregard, but one of the incoming fans should be devoted to the hood vent intake. For the hood vent setup, you should make 2 or three parallel paths instead of one series if you do have them in series (series adds heat to the next, so less heat is removed with each hop since the incoming air is already preheated, and parallel can avoid turns which diminish airflow). Also the vent ducting should be of the insulated variety. These hood changes (fresh air intake, insulated ducting, parallel) will have a relatively minor but important impact. The ducting radiation alone is enough to heat a room.
For your heat though, you will probably need an air conditioner, like a 25,000 BTU window style. I say this because its only March so you probably live in a hot part of the country and unless you've hit an early heat wave you probably have 100-110 degree grow room temps to look forward to if nothing changes. In lieu of that expensive item, you also need a vent or hose at the top of the room to suck away the hot air. Your intake fans should bring in air at floor level, and you may be better served using two as exhaust and one as intake, with one exhaust still routing through the lights but also pulling air from the rest of the room. If you have a crawlspace, air is cooler to intake from under there by a good amount. Your air mixing fans should be arranged to avoid mixing hot ceiling air if possible. If you are in a garage, insulate the roof of the garage (they usually aren't) and put styrofoam insulation sheets in front of the thin garage door. If the sun shines on the exterior of the room (like the garage door, or an exterior wall), find creative ways to shade the sun from hitting the house on that side such as a "sun shade", tarp or bamboo curtain.
If you have an insulated room protected from external sun exposure with vented lights, your intake of air should be enough assuming outside air is under 80. So there you go, lots of info but to recap the two biggest things are the hot top air venting in the short term, and sealed room A/C later with outside air venting though hoods. I assume no CO2 is used, but using it helps the plants cope with some of the heat, and if you do it might be cheaper to use once you seal off and use A/C rather than blowing CO2 away.
If you are running ten 600W HPS lamps, then your size would best be served by looking into induction (EFDL) lighting or even LED. Induction heat is low and LED is almost nothing. For comparisons, a 300W EFDL will outperform a 600W HPS easily because they're brighter and have a much better spectrum, as will LED (but they are more expensive, etc), so switching to either will pay for itself in electricity costs alone, with a ton of less heat to deal with. It really is best even if you don't have heat issues. If you are in California, its even more of a no-brainer given the cost of energy there. Just for comparison, a 400W HPS puts out much more heat with about ~20% usable light. I replaced one with a 400W induction (95% usable light) and added a 130W (listed as 240W model-- 100% usable light) LED, which combined put out much less heat than the HPS. The heat difference allows me to run the dehumidifier as needed without needing to run the A/C or bring in outside air.
Hope this helps! Good luck!
