I'm trying to find the first 3 minimum spanning trees of a single complete and all weights are non-negative.

That is finding the minimum spanning tree (which I already can do), then find the next minimum spanning tree that is greater than (or possibly) equal to the first minimum spanning tree, same thing for the third minimum spanning tree.

weight: 1st MST <= 2nd MST <= 3rd MST

There must be some change to the spanning tree (can't leave it the same).

Pseudo code would be sufficient, no need for code please, as I want to code it in my own words.