It has no official carrier branding, but I think it's safe to say that since my AT&T card worked in it and I was able to use AT&T software to doctored it, that it's for AT&T.
If the back sticker is red and says Vodafone, that's Verizon, if not, it's AT&T. The Verizon Pre3 has both, a GSM and a CDMA radio, so yes, a SIM card willl work in it
In my picture, that's a Vodafone sim card, not a sticker. It maybe possible that a prototype isn't carrier locked, no way to know without trying to doctor it with Verizon & trying to activate it.