I am pretty sure the MobilePre
(1) is portable
(2) can record two guitars at once - assuming a mono signal from each one
(3) has two mic inputs, with phantom power.
The problem is with wanting to record more than two channels at once, as it will only output two channels of audio (for example, the left and right channels of a single stereo track.) So while technically there are six audio inputs, they will be lumped together by the respective input channel numbers - all of the channel one inputs will be output to one channel, all of the channel two inputs output to the other.
So, for example, if you have a guitar in the channel one instrument input, and another guitar in the channel two instrument input, that's fine. Now where will you plug your mic? Either the channel one or channel two mic input (or both if you're using two mics) ... and the vocals will then be mixed with the guitar's signal in that same channel.
Also consider that you won't have separate level controls for both guitar and vocals in each channel. You'll need to adjust the guitar's level at your amp, or wherever the signal's coming from, so that it mixes well with the vocals coming from the mic.
A mixer (including some newer models with USB outputs) will give you greater control over your input channels, though it will still "mix down" to two channels for output to a recorder or PA system. If you're going to record three instruments and three singers, you probably want to look at an 8-channel mixer.
(4) yep; price is good. Mine was on sale for $139 at Musician's Friend - and it came with two
nice MXL 990 mics at no additional cost.