I have all three and also play the guitar.
A lot depends on your budget, time to learn, and the type of music that inspires you.
If budget is a concern the Uke will be your best bet. They are fairly reasonable. A good entry level banjo or violin will cost a lot more.
If time to learn is pretty limited the Uke can be learned quicker. The banjo is not bad to pick up but once you leave the world of frets and venture into the violin things are a bit trickier.
The violin is my favorite but is hands down the most challenging of the three.