A 2'x4' 6 bulb (324w) fixture will do very well with three plants. I've done numerous grows with that light and the same size tent as yours. Just keep it right on the plants so they grow short and bushy since there's not a lot of headroom. My best yield was almost 3oz from a plant in a 3 gallon pot, so it's definitely doable with T5. Any side lighting you can add during flower will only increase your yields since T5 doesn't have the best penetration distance.
One problem is that the heat tends to dam up under the light because it's the same size at the tent. I solved that issue by mounting a fan on one end of the light and some metal L brackets on the other end. The brackets keep the tent pushed away from the light so hot air can rise, and the fan moves air in that direction. I had a camera mounted to one of the brackets & you can see the fan at the other end: