I have seen pictures of the M2HB but not any of the M3HB.
What is the difference between the M2HB and M3HB?

There is no M3HB

It's M2HB and M3

The M3 50cal MG does NOT have a heavy barrel, and has a re-engineered buffer to compensate for this... The result is a higher rate of fire, among other things... Basically, you get M240 ROF from a 50cal platform....

The M3 is manufactured by FN, M3 info HERE

The M3HB is an aircraft M3 that has been modified into a heavy barrel configuration. Here's a couple links where you can see the difference.

www.fnhusa.com/contents/mg_m2hb.htm M2HB

www.fnhusa.com/contents/mg_m3m.htm M3

These new M3's by FN are highly modified from the originals, the older ones had trouble firing 1,200 rounds a minute. FN redesigned some of the internals so that they would last and stand up to the beating that they take.

Quoted: There is no officialM3HB It's M2HB and M3 The M3 50cal MG does NOT have a heavy barrel, and has a re-engineered buffer to compensate for this... The result is a higher rate of fire, among other things... Basically, you get M240 ROF from a 50cal platform.... The M3 is manufactured by FN, M3 info HERE

What's being called an M3HB is basically a surplus M3 converted to ground use using a combination of M3 and M2HB parts.

Here's a pic of my M2HB (on the 'pod) next to an M3HB on the bench:
M2HB/M3HB

Of course there are more 'obvious' conversions around too.   Here are some examples being made by CWA right now:
Central Wisconsin Armory

Of course, you can always try an build your own too:
M2HB.NET

While there is no actual USGI M3HB, they make them all the same.  I have one downstairs so I know that for sure.  The differences are subtle, M3 feedway is beefier than an M2, the locking block is attached differently and when you add the M2 spades and buffer it still has a slightly different rate of fire because the bolt if lighter.  ther than an extra hole or two in the sideplate, you cannot tell the difference from three feet. They are also about 1500 dollars cheaper to build than a true M2HB.