No, green poo not normally considered a problem at all.
It really does not matter - nor can anyone tell - if a baby is getting more foremilk. There is so much anxiety over this and it is usually unnecessary
Foremilk/hindmilk ratio is fine in whatever quantities for the majority of babies. There is no need to 'engineer' the timing or technique of feeding (in a healthy, thriving, happy baby) to adjust this.
Alternating the breast started on is a matter of convenience - it equalises supply (a bit) and ensures that both breasts get sim. stimulation, which can be an issue for some women who might not produce a sufficient supply otherwise, early on in bf. I don't think it matters in a happily bf 3-mth-old, honestly
Tallulah, you and I differ on this - we surely don't need to be worried about 'emptying' a breast, or deliberately keeping a baby on for longer? The pattern you describe - more milk removed on the first side, and less on the second - is normal and does not present problems. The other pattern - less milk removed on the first side - is also normal, and also nothing to worry about
OP - green poo is on the normal spectrum for healthy babies. Sounds like your dd is healthy