In addition to Noel's excellent advice, another way to get triplet notation for only certain notes is to just change the beat resolution for those notes. This doesn't change the time signature, but changes the way that notes are interpreted and displayed:

Attached Files (Click to download or enlarge) (Only available when you are logged in)
2015-01-12_11-40-29.jpg (82.34 KB, 125 downloads)
2015-01-12_11-41-53.jpg (65.79 KB, 125 downloads)
2015-01-12_11-43-43.jpg (77.23 KB, 124 downloads)

BIAB & RB2025 Win.(Audiophile), Sonar Platinum, Cakewalk by Bandlab, Izotope Prod.Bundle, Roland RD-1000, Synthogy Ivory, Kontakt, Focusrite 18i20, KetronSD2, NS40M Monitors, Pioneer Active Monitors, AKG K271 Studio H'phones