Technically your both are correct.
A Prefect wouldn't necessary be on the front line, but exceptions are made.
They'll most likely be in a command role:
Prefect mw
Principes mw
Centurion mw
2x Legionnaire mw
That's their Command Century, with another Century as support / bodyguard. But then Generals / Prefects don't really go out of their way without a fully prepared armed escort Cohort in tow, preferable with full commands and possible Naval support as well.
And your:
Legatus mw
Centurion mw
3 Legionnaire mw
Is also correct, except a Legatus is a Major, a Principes is a Captain and a Centurion is a Lieutenant in that order.
Follow this short story:
Legatus John had a cohort to run, John's Cohort. In it he had command over 3 Principes, A, B and C. Each " Captain " commanded a pair Centurions, who were either MechWarrior or Tankers. Except Principes C, who commanded Infantry. In his formation he commanded 2 Centurions who in turn commanded a Legionnaire who also resided over several Miles Gregraius, " Sergeants " who then command several Miles Probatus, " Corporals " who then command the rest of the troopers made up of Miles, " Privates ". Also as a side note, Principes A's and B's Centurions would have command over Legionnaires, again, MechWarrior or Tanker.
TT