One thing you will need to understand is, having a 3-year undergraduate degree is not the end of the world. So, just relax. Now that I have got your attention, let me explain.

Like Spartacus and skafle suggested, there are sill universities here that will not discriminate you for taking less credits in Nepal to complete your undergrad. Instead, they will put you into some courses, you may call them MBA pre-requisites. Then you will officially begin your MBA once you earn those credits. If you do the math, the number of credits will be close to what Spartacus just said, but again, this is all at the discretion of the university.

On a side note, having your credits evaluated from a service (such as WES) will allow you to transfer your credits to a 4-year university and you can work toward getting your 'American' undergraduate degree. But, I'm guessing if you have the option to enroll into a Masters program, why would you even consider enrolling into an undergraduate program? That being said, work hard to find the colleges or universities in your area that match your interests, field of study, affordability, among other things.

Good luck to you.

Jay Nepal!!