If you are indeed doing your rotations, have shield oath up, are properly geared and are using defensive cool-downs, it sounds more like a healer issue to me.
Frankly if you have an overly geared pt, and esp blm/brd combo, 3 mobs is a waste imo since they can be burned down incredibly fast in quantities even larger than 7. Its incredibly frustrating as a dps to see tanks, like the one I had in wp hm tonight who is geared to the teeth (this guy even had his zetas) but are too scared to pull more than 1 small group. Though I am pretty sure the tank I had tonight was duo-boxing the other dps and there is no tank worse than a duo-boxing tank. No matter how good you think you are doing it, you're not. But I digress.
I would say read some tanking threads, maybe get a couple friends to run a dungeon or two with you to try new techniques and practice mass pulls. That's how I learned to heal larger pulls and when its safe to dps as a healer and when to hold back. It just takes practice. Also, this is something ppl forget to do constantly and its the most obvious, read the ability descriptions so you know what they do. Never make assumptions that just because B was learned after A that its better or more powerful.
Also, unlike what a previous commenter stated, the group sets the pace for the dungeon, not JUST the tank. Do what your pt as a whole can handle. No one likes a smug tank.
