Oh boy my friend your opinion of yourself inhibits you to perform. Every man has the problem that he fears of failing, he is stressed and the more you fret over if it will work this time, the more you will fail. Guaranteed! I have no advise in how to relax and ban the worries from your head. But if you go into it, free of fear of failing, I am sure you find a difference. I have never experienced a 3sum but if your partner is not completely behind it as you have to be, i am sure the emotional baggage can be long and damaging. Try games, role play or simply a light bondage. Again, it is to stimulate her, get her to the point where she is so ready for your penetration, so wound up and almost there that A: she is wet enough and B, it wont take long. Still try lubrication so your contact is not as stimulating and maybe using a condom reduces your sensation to the point that she can cum first.
But most of all, try to ban the worries from your brain and go into it not thinking of your failure but how every touch makes her feel, every kiss on every part of her body makes her reach a new high. Maybe you find my blog on light bondage, read it and try.
Again best of luck