Commend to your attention Cmdt Saski's three volumes on 1809, especially for the first portion of the war. Available on Google Books, for example: https://books.google.com/books?id=NkxBAAAAIAAJ&printsec=frontcover&dq=editions:Qwkownxe2LMC&hl=en&sa=X&ved=2ahUKEwif5-Swzp_kAhVNrVkKHbCrDqkQ6AEwAXoECAUQAg#v=onepage&q&f=false

For the period just before the war (15 February to 1 March 1809), the 3e Léger was recorded with the following "présens sous les armes" (from 'Situation Générale de l'Empire' for that period, SHD/11C179) by battalion (officers/men):
I: 28/786, with the "Corps d'Observation" that would become Massena's 4e Corps
II: 48/766, with the "Corps d'Observation" that would become Massena's 4e Corps
III: 15/549, ordered to complete itself and move to Piacenza/Plaisance--it was to join Massena's 4e Corps, but was one of the battalions under Gen. Bisson that was trapped and captured in the Tyrol (as we have been discussing recently).
IV: 665 (total, not listed by officers/men) with 7e Corps of the Army of Spain.
V: 10/255 at the depot.

With apologies, you may also wish to look at Thunder on the Danube.

Note, however, that the report above and the standard 'Situations' submitted periodically by each corps d'armée only include senior officers at best. That is, regt'l colonel almost always and (usually, but not always) battalion commanders.
So Saski (and Thunder) will give you overall regt'l strengths, but not names of officers.

The 78e de Ligne carried on the traditions of the 3e Léger, but not clear that a decent history was ever published.