I ran a simulation with my broker only and when I parsed the state file I can see there are 3 broker in the simulation.
What is this LMP broker? Can anyone explain please?
As Govert said, lmp is the wholesale provider. The name reflects the idea that it's supposed to approximate the behavior of locational marginal pricing in a local market. But there should also be one other broker called "buyer" - this is another fixed player in the wholesale market that provides liquidity - it always tries to buy some energy in each timeslot, generating bids that range from large quantity/low price to small quantity/high price. The reason this is needed is to ensure that there is always a buyer in the market even if all the retail brokers are trying to sell.
The reason these all show up as "brokers" is that from the standpoint of the wholesale market, all participants are brokers. But some brokers also play in the retail market - those are "retail brokers" like SPOT and default broker.