The 1990Philippine Basketball Association (PBA) Third Conference was the third and last conference of the 1990 PBA season. It started on September 30 and ended on December 20, 1990. The tournament is a two-import format, which requires each team to have two American reinforcements.
Teams in a group will play against each other twice and against teams in the other group once; 10 games per team; Teams are then seeded by basis on win-loss records. Ties are broken among point differentials of the tied teams. Standings will be determined in one league table; teams do not qualify by basis of groupings. 
The top five teams after the eliminations will advance to the semifinals.
Semifinals will be two round robin affairs with the remaining teams. Results from the elimination round will be carried over. A playoff incentive for a finals berth will be given to the team that will win five of their eight semifinal games. 
The top two teams (or the top team and the winner of the playoff incentive) will face each other in a best-of-five championship series. The next two teams will qualify for a best-of-three playoff for third place.
Purefoods Hotdogs and Presto Tivolis earned the finals berth playoff incentives by winning five of their eight semifinal outings and are locked in a three-way tie with Shell for the second finals berth. Purefoods gets the #2 seed via higher quotient. Shell and Presto will play in a knockout game to determine who will face Purefoods in a finals playoff match.