Don't worry - its the right place to post.
I guess you mean Prime Express 3 vs. Mathcad 15.
The communication here is very limited, better said, nearly nonexistent!
- MC15 can't read Prime files and there is absolutely no way to convert/save a Prime sheet to MC15 format.
- Prime won't read Mathcad 15 sheets either, but there is a way to convert legacy sheets to Prime format. You need a full installation of a current Mathcad 15 to be able to do so.
So if your school still uses Mathcad 15 (which is a good decision anyway), it would be adviseable to use that version, too. There is a trial version of Mathcad 15 to download, too, which will work for 30 days as will the trial of Prime. The difference is that after this trial period Prime will still work but in a very limited Express mode (no symbolics, no programming, no solve blocks,..) while MC15 will not work at all.
If you have a student version of prime with a valid product code and you got your license file during installation, then you can install MC15 and use the same license file (not the product code) for it. In other words, if you are entitled to use Prime you are also entitled to use MC15.