Lubricate!
People put body glide on the edges of the wet suit, this may also prevent chafing at the neck.
I myself have also had a lot of chafing in the nether regions after a 70.3. I got the advice from a friend to put lube down there, helping him to completely prevent the problem.
For the swim you may opt to roll down your tri suit to the waist, then put your wet suit over it, and dress again after swim exit. You also see pros do this a lot of you watch for T1 footage, probably because it reduces restriction of the arms. I have tried it myself but it takes a lot of practice to get into a wet tri suit