Joint papers as PhD student During PhD studies (math/applied math) is it considered ok to have mainly or solely joint papers with other authors? For example three-author papers, me, supervisor and another author.

Can such papers form the basis of the PhD thesis? My concern is whether if only first-author or single author papers are considered useful for PhD studies.
It will depend on the requirements of your course - ask your supervisor for clarification.

Having said that, I did write a handful of papers while I was completing my PhD - not a single one was with me being a sole author. Three of them had my supervisor and another academic as co-authors and the other 2 was with another academic (I was co-author of these).

These papers formed the 'backbone' of my successful PhD thesis. My supervisor indicated that having the papers published and then referencing them in the thesis served to strengthen the thesis, as those points had already been peer-reviewed.

If anything, they demonstrate some ability to collaborate with other academics.