There isn't really any such thing as a beginner rep, of course there are more specialised reps but if you do plenty of research before hand you will be fine with most lizards. BD's aren't as easy as they're made out to be as they require constant temperature monitoring amongst other things but they're certainly not difficult if you know what you're doing. Personally I'd choose the rep you want, research and cater for its needs as apposed to picking a rep to suit the viv you have.
A 3ft viv isn't great for a BD at any age as they require a high temp basking area and a cooler area to regulate their temps. This is hard if not impossible to achieve properly with such a small viv and either the basking temp or cool end temp will be hindered which can cause serious issues such as not being able to digest food properly or dehydration. If you're looking at BD's you're better off purchasing a 4x2x2 straight off, it will have to happen anyway and its not worth compensating the welfare of the BD in question.
So to answer your question if you're set on using the 3ft viv then I wouldn't recommend having a BD, if you're set on having a BD then buy a 4ft viv.