Sparse Matrix problem

Any hints on solving linear systems of equations using the Isolve function?

Yes, look at the built-in help. You may also search this forum.

I have a problem with 5 equations but with 3 unknowns. Any clues?

Clues for what? You probably know yourself that you cannot expect a solution in every case if you have more equations to satisfy than variables. Maybe something like minerr() could be appropriate.

If you have a concrete problem, state the version of Mathcad used, post the worksheet to show what you have done so far and clearly describe what you are searching for.

In Mathcad 15 lsolve for overderdetermined system gives "pseudosolution", i.e. solution with minimal norm

For underdetermined system lsolve gives: