Common, you must be acting like...............to ask about scope of nursing in US? Possibly there is no way to transfer any of your Nepal credit to US college, as per my understanding but if you start here then you can transfer from one univ to other easily for some extent. There is no way you can go for graduate school in US from Nepali 3 yrs IN  certificate.
If you are already in US then continue your undergrad or start the process of NCLEX_RN to get job asap. If not then its not a bad idea to complete US undergrad in Nursing.
Good luck

Since you already have a staff nurse degree in Nepal. You can either apply for NCLEX  and because yodidn't go to school in US you have to qualify for English proficency test TOEFL and CGFNS visa screening before you can take NCLEX. This process might take atleast 5-6 months because the board of nursing has to evaluate your credentials and transcript from Nepal.  But even if you pass NCLEX you need to have SS# to get NCLEX-RN licensure. I don't think any university would transfer any of your credits from Nepal so you would have to start from the scratch take all pre-req and apply for nursing program for undergraduate. Like ajneya said you can't go for grad school since you  need to have 4 years bachelors degree to go grad school and some work experiences.