A bass player I played with for a while showed me this neat trick. Thanks Lenny!
First write down the notes in a row like this. I’m starting with C because that’s the key I want to transpose the chords from.
C C# D D# E F F# G G# A A# B
Now what key do you want to transpose to? How about E? If that’s the case, just grab that line of notes and rewrite them under the first line- only the E will be under the C.
C | C# | D | D# | E | F | F# | G | G# | A | A# | B
E | F | F#| G |.G#| A| A#| B | C. |.C#| D | D#
Do you see? So let’s say you want to transpose C,F and G to the key of E. What you’re saying is “if C is now E, then F=A and G=B. All you’re looking for when you transpose is the same relationships in a scale with a different root so 1/4/5 in C is C/F/G which relate to E/A/B in the target key. Hope that helps you!