Senegalese striker Gueye, 26, moves to Ewood Park for an undisclosed fee on a contract that runs until the summer of 2027.
He began his career in France with Saint-Etienne and then Nancy before moving to Belgium to join KV Oostende four years ago.
The 6ft 5in forward signed for Molenbeek last summer and scored 11 goals in 32 league games as they finished bottom of the Belgian top flight and were relegated.
Japanese Ohashi, 28, also joins for an undisclosed fee, having scored 15 goals in 27 games in all competitions for J League side Sanfrecce this term.
He joined J League side Shonan Bellmare in 2018 and won the J League Cup in his first season, going on to make more than 100 appearances for the club over five years.
Frenchman Mafoumbi, 19, signs his first professional deal having scored 50 goals in his past 72 games in youth football for Nantes.
The Championship club have the option to extend all three players’ deals by a further 12 months.
Blackburn start the new season at home to newly-promoted Derby County on Friday 9 August.