the only seasons which have broken that particular formula - and which featured teams of 5 Rangers or more - are Ninja Storm, Dino Thunder, Jungle Fury, and RPM, all of which began with 'core' teams of 3 Rangers each, and slowly added the other Rangers over the course of the season.
I wouldn't count RPM in that. Sure, there were only three active Rangers in the first episode, but the characters who would become the fourth and fifth Rangers were there from the premiere onward and became Rangers in episodes 3 and 4. And it was established from the start that it was intended to be a 5-member team; they just hadn't finished filling it out yet.
So RPM was definitely the type of season where the core team is 5 members and is then added to later on (when the Gold and Silver Rangers show up around episode 15). The other three had 3-member core teams that were added to gradually:
In Ninja Storm
, the Wind Ranger core team was 3 members. The Thunder Ranger twosome were originally introduced as rivals and the teams didn't combine until episode 12 or 13. And a non-Ranger regular, the team's tech guy, became the sixth Ranger several episodes after that.
In Dino Thunder
, you had a 3-member core team with Tommy (from MMPR/Zeo/Turbo) as their mentor for the first 4 episodes, then Tommy became a Ranger again in episode 5. Then the White Ranger showed up in episode 11 as an antagonist, and it wasn't until episode 21 that he reformed and brought the team complement to five.
In Jungle Fury
, you had a 3-member core team for 15 episodes, the longest stretch of time that the team has remained that small. Their mentor RJ became the fourth Ranger in episode 16, and the fifth Ranger didn't show up until episode 19.