Can you use a triple front shifter on a double crankset?
 

I have a FD-R443 flatbar mtb to road sized chainring front derailleur. I am thinking I would like to use it with my double crankset. is there a problem doing that or do I have to get the double front derailleur?

It really depends on the chainring sizes. Most Mtb triple derailleurs are designed around outer chainrings of 46 or 48 teeth. If your road bike has a 53t outer ring, you might not be able to mount it at the proper height without the back of the cage touching on the outer.

The FD you referenced is meant for road chainrings and flat-bar/MTB shifter, so that sounds fine.

You should be able to use it with a double crank just fine; lock out the third shifter position by setting the FD limit screws.

The FD-R443 is designed to use road bike chainrings, I believe its top capacity is 52 or 54t. it is also for triples. But it works with the amount of cable pulled for MTB shifters. I am building a road bike frame but the frame doesn't have any down tube shifter bosses and I don't have the proper brifters so i think I am going to have to use an 8x3 MTB shifter set and put a flat bar on it for now until I get some STI brifters or bar end shifters.

I just happen to have a double crankset (52t and 39?) and I wondered if I could use the front derailleur I already own.

Set up the limit screws normally, then I'd suggest that you set up your shift cable such that the large chain ring is engaged when the shifter has fully taken up cable. This way, if you try to make an "extra" shift down you will be slackening the already loose cable more, rather than trying to shift up against the high limit and possibly overstressing the cable or jamming the shifter ratchet.