The big chain ring on your triple is the same as what's here. And the difference between a 39 chain ring and a 32 chain ring is not enormous. It will probably work well enough. I raced for a while on a 10 speed chain/cassette and a 7 speed crankset. Sure, I dropped chains, but not all the time, and I got some nice compliments when I didn't panic and looped that chain back on by moving the derailleur while keeping my line in the middle of the pack. Then I just raced exclusively on the 53 until I could afford to upgrade.
I think I told you to get a triple front derailleur in another thread, but you could probably just fudge it and see what happens. It comes down to whether you want to follow the manufacturer's recommendations or just make what you have work and save $25. I've definitely come down on the saving money and making it work side often enough.
Double derailleurs will actually work as triple derailleurs without modification. The amount of "pull" on the cable is the same for one gear click.
That being said, there are a couple of things that will not be optimal:
1) The chain will rattle against the cage in the extremes on the inner gears. So, you'd have to trim the front more often.
2) The chain may drop more often when shifting to the granny.
3) The double derailleur may not have the range to get to all three chainrings, depending on the bottom bracket width. Sometimes the granny is too far in and the derailleur cage hits the seat tube before the correct position can be found, or if the outer chainring is too far out the cage may not be able to get to a position where the highest gear doesn't rub.
Try it and see is the only advice I have. If it doesn't work, just getting a Sora 9-speed (3403) is only about 25 bux new (the difference in front der's between product lines is minimal). Ebay the Ultegra for about 20 bux, and it's a wash. Just changing the cage would be difficult, plus you'd have to find somebody with just a cage for sale...
A bigger problem is the rear derailleur- a short-cage double won't have the capability to wrap enough chain to allow both the big-big and little-little combo's on a triple. Assuming a 23/11 cassette, 53-32 + 23-11 = 33 teeth, and the max capacity of a SS short cage derailleur is only 29 teeth (maybe 31 if the stars are aligned and you don't care what Shimano says). If you want to insist on using the SS cage derailleur, then it would be best to make the chain just long enough to accommodate the big-big combination, since a chain that's too short for the big-big can snap when shifted into a larger gear, but a chain that's too long for the small-small will just hang loose (still not great, since it can jam).
Works fine with friction, but you can forget nice clean shifts with STI levers.
And using a newer triple front derailleur won't work well either! The newer triple front derailleurs are looking for a 39t middle, not a 42 so the inner cage is deeper. Set it up like a normal FD, and the inner cage will hit the middle ring, but raise it up to clear, and you end up with poor big-ring chain control and the attendant thrown chains. STI front shifting depends on parts designed to the task, and any compromise will equally compromise shift quality.