Keni Lee Burgess and others have talked about string relationships. It took my hard head a long time to understand what they were talking about.
Trying to keep it basic, if you can play a 3 string tuned GDg then you can play also play a DAd, EBe, or CGc. The string relationships are the same, but the tone and pitch is different.
If you can play a GDg you can also play a 4 string tuned GDgB. To start, just think of it as GDg but on a full strum, play all 4. You get a bit more treble and it is a usable thing. Furthermore, you can do finger chords on the 3 smaller strings -especially if it is a fretted guitar.
If you can play a GDg, you can also do some fun basics on a six string in open tunings. In open D- the 3 bass strings are DAD- same relationships as a 3 string. Open D tuning is DADF#AD.
Open G is another tuning that a lot of folks play. it is DGDGBD. Some remove the big D string. I do better without it.
Use the 3 strings that are the same relationship as a 3 stringer and give it a go.
Additionally, there are finger chords you can use. Note and melody picking seems more instinctive on the open tunings.
You can look up open tuning and finger chords on line.
If you are totally happy with a 0ne, Two or Three string, then God bless you. Keep at it.
If you want to broaden your horizons and try new things-- do it. A lot of what you know on a 3 string will work on a 4, 5 or 6.