What boat would have a sail that measures 33ft x 37ft x 12.5ft?

That's impossible to guess. Do you even know what type of sail it is? Is it someone's mainsail, mizzen, jib? What weight the material is and even what type of material? The suggestion that you stretch it out, photo it and include dimensions when you list it on a boating parts website might get you a response and maybe even an offer. Do you know its age? How much use it's had? I wouldn't give you $10 for a sail that might be worn out. Remember that a person's life can depend on the sails functioning properly under the conditions for which they were designed.

Does it have a number or an emblem on it? Battens? If not it is probably a head sail If in good condition It would be worth listing it on a sailing site. Incidentally if it is a headsail then all 3 dimensions are needed.

It doesn't make any difference as long as it fits. Basically a sail is a triangle so I don't understand why you have 3 dimensions. The 12.5 ft is the length of the boom. The only other dimension you need is the distance from the boom (at the mast) to the top of the mast where the halyard exits the mast.

That is like asking about the vehicle that a 33X9.50X18 tire came off of - Just take a picture with measurements, including any grommets, or mast sleeves and post it on some large sailboat websites in the classified area.

If there's a sail number than might identify the boat - people register them for racing

Sounds like a jib sail for a three master.

Not to be a jerk, but if you need to ask perhaps being on a boat is not the best place for you.