[Branjovi]

what the last guy said.

the major 3rd on a guitar, when fretted along with the root, is sharp. this is due to tempered tuning/intonation. google it. also google "just intonation".

you can get around this by learning to play properly with experience. learning how to play involves playing with others in band situations and learning what voicings sound good and which ones don't.

also why certain notes sound good/sweet and other sound harsh/nervous.
wonder why blues players always bend up to the major 3rd?
the fretted maj 3rd is sharp. so is the flat seventh. so is the minor 3rd to a smaller degree. and the 9th.

there are ways around this. listen to Bill Frisell. he plays with a lot of just intervals.
for more on this also check out Steve Kimock.

unless you are playing jazz or playing cowboy chords, don't play 3rds in your voicings unless you have to. especially with distortion.
let the melodic voices dictate the major/minor quality.

you can play just intonated major 3rds on guitar using natural harmonics but that is a whole other can of worms.