Pick two or three from your stash and find patterns that use them all - beta
Switch on Multi-Yarn Matchmaker in Settings → Appearance → Beta Features, then open the Discover tab, where it now has its own entry. The ordinary Stash Matchmaker is unchanged and needs nothing switched on - this is the version for combinations.
Browse or search your stash and tap the yarns you want to combine. Two is the minimum, three is the maximum, and the screen keeps count as you go, showing what each one has available.
Tap Find Patterns and it looks for multi-colour patterns - colourwork, stripes, anything written for more than one yarn - that suit the combination you picked. Tap any result to open the pattern.
Nothing found is a real answer too: three yarns of unrelated weights have few patterns in common. Drop one, or swap it for something closer in weight, and try again.
Available on iPhone, iPad, Android, and web at app.stash2go.com