3 month is rather short to get any shrinkage, but your T production should have stopped by then.
So, during the rebound period, your fertility and T levels could be impacted for a while. Its possible that your T level and fertility (sperm count) may be different (but in theory should not be too different) after than before (not enough study to be sure of this). But, there is a definite link between phytoestrogens in the environment and decreased fertility so I'm guessing estrogens have a similar effect.
Very little has been studied about what happens after a prolonged high level HRT stoppage.
Say more than 1 year on HRT. I don't think the testes sterility or stoppage of T is permanent; though, it may not recover fully.
They shrink but otherwise their function is not altered, this means that once E is stopped
the pituary gland will simply tell them to restart production since the body needs E or T,
restarting male puberty. Don't know if this restarting is as efficient at a later age than
at puberty, but it does work and eventually you get a substantial amount of T in your system and
produce a X level of sperms. IS T and X the same as before, probably not.
Even if your sperm count is low, you could still get a child after a year,
just at lower probability. And,
the last resort would be in vitro.