Well, 3foot would be "okay" for a young beardie, up to the age of about 8 months... Then he/she will NEED to be transferred to a 4foot long enclosure.
My advice, start with a 4x2x2 then you won't have to stress the animal changing enclosures... (For what it's worth, 4x2x2 is "minimum" size requirements for one adult Central/Inland Bearded Dragon.)
Hope this helps!
Best,
Carolyn