Hi, L.,
Sorry I'm late responding here, and maybe you've already made a decision... Anyway, I have had a couple mammograms while still breastfeeding. I was down to nursing twice a day, though, and my girls were getting most of their nutrition from other sources. So I was no longer getting that very full feeling in my breasts that you get when you're nursing an infant all the time, which you are still probably experiencing with a 3-month-old.
At any rate, I was instructed to bring my pump and to empty my breasts before the mammo, and I definitely recommend you do that. Better yet, could you bring your baby with you, with your husband or a sitter -- and nurse at the office before the mammo? For mine, the tech was a little surprised both times, because it is pretty unusual, but if you need to do it you need to do it!
I had actually had a baseline mammogram at 35, before I had kids, and I didn't find these later mammos to be any more uncomfortable than that one. So the breastfeeding didn't make it worse for me. It is not a fun procedure, but the compression for each image lasts only a few seconds. If you've gone through childbirth, you can handle it!
Good luck,
A.