Right. It's pretty much an unwritten rule to mix-up your Jabs.
Though I would say it's also because not all three punches have the same start-up/recovery for any character. Throwing the third jab makes the character slow to recover...and if that character missed or had his attacks shielded, the slow recovery can open that character up to a counterattack.
It's the same reason why I never use all three Jabs all the time with ZSS. Her last two jabs have incredibly slow recovery. The same applies to Ike's third Jab, I believe.