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What does that 3 with lines stand for?
It's a triplet. It means that you should play the three notes indicated in the amount of time that you would normally take to play two.
This means that the three crotchets/quarter notes take the same time to play as the one minim/half note below.
It means that you play the three crotchets (quarter-notes) in the time that would usually be taken by two.
Or to put it another way, the B F# G will take the same time as the D# underneath.
It's called a triplet.
I assume this piece is in 4/4 time. The -3- indicates triplets. Specifically quarter note triplets.
Starting on beat 3 you should play 3 notes spread out over 2 beats.