Here's a link to reviews from several people who have been to all three.
http://chris-n-cheryl.com/v-web/bulletin/bb/
I've personally been to SEPC and Capri (twice). There are things I like better at each. I've posted a very long review including specific comparisons on the link above.
It's important to note that these resorts are MUCH more similar than they are different. Of the three, SERC has the worst beach but it's the newest with all the bells and whistles. Of the three, Capri is the only one with one-person Jacuzzi's in the room, but it's the smallest and therefore most people say it's the most 'intimate' and personalized. Of the three, SEPC is the only one to win the 'best pool in the Caribbean' award; but it's in the DR where reports of sickness associated with food is higher.
However; in general, you can expect that the service and amenities will be nearly identical at each.
You really can't go wrong with any of them (imho). Let cost and your personal priorities make the decision for you :)
Cheryl :)