Those three South Americans go fine together (some like equal proportions, I prefer 50% Brazil, 25/25 of the others). Either chuck them all in together and pull a touch before 2nd crack (only a tiny touch b4), or roast the Brazil separately, and take it into second crack (CS10 ish) and do the others a touch lighter... 2 to about CS9ish.
Pity you haven't got some of the Sulawesi Toraja, coz I've got a blend of 50% Brazil, 25% Sula, 25% Gambella which is fantastic as a flat white (perhaps my favourite of all I've come up with). Also works quite well in Aeropress, and as an espresso.
I've tried all of the above as SO lattes, and I like all of them except the Columbian.