These fitting are sealed by the o-rings on the outer wall of the tubing, not the threads. Teflon tape and pipe sealant will not help. If you have replaced the o-rings, the problem is most likely the tubing ends.
I would try trimming back the ends of the tubing. Take about 3/4 inch off so you use a new sealing surface. It is important to get a nice smooth, square cut so use something very sharp. Be sure the outside surface of the tubing ends are clean, free of dirt, and without scratches and nicks.
When you assemble it, use a very small amount of a food grade silicone grease to lubricate the tube end so the o-ring sits properly. I use Molykote 111, but there are others suitable as well. The grease is there so the o-ring does not roll, or get pinched, not as a sealant so just a tiny smear is all that is needed.
If it STILL leaks, then new fittings are probably needed. Especially if they have been over tightened they can crack and not seal properly.
Annapolis, MD, USA