It's either an optical printer conversion, typically you cut the negative, time for an answer print, then knowing the printer light values, make a contact-printed timed IP from the 3-perf negative, then you put that in an optical printer and create a 4-perf 35mm dupe negative, from which you make a print.
Now of course it is possible to put a cut 3-perf negative into the optical printer and create a 4-perf 35mm print directly, but that is less common, both because you put your negative at some risk using a spliced roll but also because all of the printer light corrections would have to be done in the optical printer. Today, you'd have a hard time finding a lab to do that.
And there is the other issue of getting the negative cut and whether you'd have to make it single-strand (so any fades and dissolves would have to be done in an optical printer and dupes cut it, or done digitally and recorded back to 3-perf and cut in) or A-B roll cut, which then means you would have had to pick standard lab lengths for the fades and dissolves, plus now with an A-B roll negative, you can't easily do a single step blow-up from a 3-perf negative to a 4-perf print in an optical printer (which as I said, was already unlikely).
The more common approach today is to do a D.I., get the negative scanned, color-correct digitally, do all the transitions digitally, and then use a film recorder to put it out onto a 4-perf 35mm negative, then make a contact print.
As Tyler said, the 3-perf image area is "super" and the 4-perf 35mm print projection area is smaller, within "Academy" width to allow a soundtrack on the side of the print.
Which of course brings up sound...
All of this seems rather complicated just to make a print for a short film. If your intention was to make a print, you should have shot it in 4-perf standard 35mm (1.85 or anamorphic 2.40) so that you could cut the negative and make a contact print.