What are the consequences of a recommendation from someone in the distant past?
If research is a substantive part of the jobs you are applying for, then you should be wary of asking your Master's supervisor for a letter.  It could work out OK, but there's a serious risk.

The issue is that hiring committees want information about your current research, rather than what you did for your Master's degree.  If your Master's supervisor has read some of your recent work and is capable of discussing and evaluating it in detail, then it will be a useful recommendation.  However, you don't want a letter that focuses on the past and just includes a brief paragraph saying "And the Ph.D. research seems to have gone well, but I don't really know anything about it."  That could actually hurt your case by suggesting that you couldn't find three people who understand and think highly of your more recent work.

A strong letter that focuses on the past is certainly better than a weak letter, so it could be your best option.  You might also talk about this issue with your Master's supervisor, and see how compelling and up to date a letter she thinks she can provide.  If she has read or wants to read your recent work, then that's great; if she is hesitant about the idea of writing a letter, then I would ask someone else.