Hello @rosdav and welcome to the community.
What a great situation to be in and playing around with triads (plus 6ths, scale fragments, etc.) is a fine way to add a 2nd guitar part to singing / strumming by others.
It is most useful to know several things here:
The Note Circle: https://www.justinguitar.com/guitar-lessons/the-note-circle-mt-101
If you know the chord progression by chord shapes and think of their names based on the root notes of those shapes, the note circle can be used as a quick reference. You count clockwise as many jumps as are needed to match the capo position.
Example:
chord progression (by shape):
| C | Am | G | Dm | F | G | G7 | C |
capo position: fret 3
using the note circle and counting 3 clockwise jumps:
C → Eb
Am → Cm
G & G7 → Bb & Bb7
Dm → Fm
F → Ab
The chord progression in actuality is:
| Eb | Cm | Bb | Fm | Ab | Bb | Bb7 | Eb |
Another approach is to know the chords in a key, along with major scale kwedge across multiple keys, and be able to label chords using Roman numerals. Major chords are assigned upper case and minor / diminished lower case.
Again, starting with the chord progression (by shape):
| C | Am | G | Dm | F | G | G7 | C |
| I | vi | V | ii | IV | V | V7 | I |
With a capo at fret 3, C becomes Eb. You need to know the notes of the Eb major scale:
Eb, F, G, Ab, Bb, C, D, Eb
You can then list the corresponding diatonic chords:
Eb, Fm, Gm, Ab, Bb, Cm, Ddim, Eb
You can then use the listed progression in Roman numerals as your reference tool.
I have written a tips guide on using a capo to stay in the same key which may provide some useful insight even though the capo use you ask about is keeping the shapes the same and changing key.
I hope that helps.
Cheers
Richard 