You are asking a very open question, might need to be a bit more specific if you want to stir up discussion on these 2 in the same thread.
Tren - Has an effect on IGF, synergy with your HGH, pulls IGF into the muscle. I have a hard time justifying an HGH dose over a few IU without at least a low dose of Tren next to it. Other than that, we dont "need" it but there sure are some pros if you are willing to make the trade for the cons.
T3 - Personally I am a fan, don't need to go crazy but why not enhance your metabolism to assist in the overall picture. I use this year round, usually at 75mcg these days and my TSH is in range on that dose (fast but in range). I am using 100mcg on a growth phase right now.