Lampard dispensed with the 3 at the back after his first 2 games in charge because CMs became available to him like Delph and Donny VDB. He then went to 4 at the back from then on until the Chelsea game at home, then he stuck to that for the last 5 games.
After starting at Newcastle with 3 at the back, he set up with 4 at the back against Leeds, Southampton, City. Then we played Spurs away and got mauled with 4 at the back and he dropped the system for 3 at the back against Wolves at home. Then he went back again to 4 for the win against Newcastle at home. Stayed with 4 at the back against WHU, Burnley, Man United, Leicester at home and Liverpool, then reverted to a 3 for the Chelsea game, Leicester away game, Watford, Brentford, Palace and Arsenal games.
This is not a manager wanting ideologically to play 3 at the back. He went there because of what was available to him in midfield...if he thought the 2/3 CMs could do a job in front of the defence, the system was 4 at the back.
This will be seen again to be the case once Gueye and Onana are in the team. Regardless of Coady - who in any case will play in a 4 and has done before.