CPU support:
6VX7B till 1,22 Ghz Tualatin (multiplier till 9,5)
6BX7 till 1,22 Ghz Tualatin (multiplier till 8 )
6VTXE full pentium 3 support upto tualatin 1,4 Ghz (multiplier till 9,5)
all 3 have same FSB support upto 150 Mhz
Memory support:
BX7 upto 768 MB pc-100 support (3x)
VX7B upto 1,5 Gb PC-133
VTXE upto 1,5 Gb PC-133
chipset:
6VX7B VIA Apollo Pro Ultra ATA 100/66
BX7 Intel 440BX IDE UDMA 33
VTXE VIA 694T/686B Ultra ATA 100/66
The VX7B and VTXE both have a "shared" ISA iirc and the BX has 2 isa slots
the rest is pretty much the same or very similar, these are to me the most important details which to decide the choice on. personally for a Socket 370 build I would choose a intel chipset over a Via chipset, but the via chipset offers its advantaged in terms of memory and UDMA, but the BX board has 2 isa slots if you value that higher though. The VX7B would fall of for me since its the least desirable of the 3 in terms of features, the decision is hard to make imo.