Bit late to the party here. After reading a good many replies, my advice, as both guitar and bass player, is, first: commit to using all four fingers; you will definitely be limited playing only 3-finger/3-note chords (unless you are doubling on some strings or using barre chords exclusively?; not sure what you mean by 3-fingered guitarists). I would doubt that your hand is really weak, but because you will be using it differently, it will initially be frustrating, no getting around it. Second, figuring out tunes on guitar depends largely on knowing chords and keys, not so much following root notes or the outlines of chords, as with bass. You will learn melodies, of course, but that also develops out of chords, keys, and scales. Finally, and most important, find a good teacher, someone with solid teaching experience, not just a guitar player who can show you a few things, and commit to the learning experience, whether that is chord study; reading, and playing from written music; or what have you. Of course, you can learn on your own, but why put up with the frustration that comes with trying to follow a book (to which you can't address the many questions you will have) or what are usually very slow, and also unresponsive, online/video courses that are, most often, trying to sell you something. A teacher sitting in the same room with you is essential. Learning to play guitar is challenging--but definitely rewarding. I hope this helps.