You really need a min of a 4' viv for a beardie, 3' is just too small.
Just to throw a spanner in the works, What about a rankins dragon. Same temperment as a bearded dragon and pretty much the same care. They only grow to around 12" in length and would be happy in a 3' viv.
check out this site for pics & more info Rankins Dragon .co.uk
Sorry i know nothing about MHD's so can't help there.