Are the motors both the same frame size? If not how was the motor mount modified to fit the 3HP motor?
I'd like to see a picture of the sheaves to determine if they're worn.
I don't think 37 1/2 c-c is a long belt, I've worked with drives that are way longer than that. Shorten the c-c dimension and you have problems with wrap on the sheaves w/o changing sheave diameters.
I googled a picture of the jointer and don't see how you'd shorten the c-c dimension of the drive without major modifications.
I'd buy some new brand name belts and make sure they're installed properly and tensioned maybe a little on the tight side and give it another go.
Confidence: The feeling you experience before you fully understand the situation