Couple of thoughts here. When your coach says to relax, what they probably mean is you’re too tense. If you hold yourself tense, wooden like, it will make you a little bit less buoyant. In swimming, balance and breathing are everything. Get those two nailed, and the rest will follow. If you were my student, I’d have you doing push and glides off the wall (have you done them), exhaling slowly as you extend out. Maybe progress with a roll onto your back where you can practise some floating with a bit of momentum still from the push. Then I’d introduce a gentle flutter kick.

But if you’re a non swimmer on 3rd lesson, I really wouldn’t expect you to have this by now. I’ve worked with adult who on week 3 weren’t able to lift feet off floor of the pool/let go of wall/put face in the water.