My 3 cats who have loved each other have suddenly started attacking each other, what is going on?

I have 3 indoor cats... 2 (a male and female from the same litter) are 2 years old and one male who is 12 years old. they all love each other and sleep together and so on. but last night the strangest thing happened. A randomn cat came to my screen door and my 3 cats freaked out... so I sc ared away the random cat an everything was fine. but 3 hours later after i went to bed I woke up to the sound of a horrible cat fight. I ran into my kitchen to see my 2 males tearing each other apart. After a difficult struggle I got them separated and locked up the younger male in a separte room for the night. I went back to bed and was awoken again by s snarling and growling sound... again I ran to th kitchen and my older male and my female were face to face snarling and ready to attack each other. so I immediATELY brought my older cat into my bedroom shutting the door. I then let out the younger male to have free roam of the house with his "sister cat". Again about a half hour later I awoke to them snarling and screeching and fighting! I thought , what the heck is happening! I was so upset. I then put ech cat in a separte room isolated from each other for the rest of the night. then this morning I was hoping everything would be back to normal and I let them all out into the main living area and again they immidiately went after each other, in a severe cat fight. This has NEVER happened with them before. They have never shown aggression towards each other. I am so confused I dont know what to do. they cannot interect at all with out a fight breaking out. Its like I am in a crazy scary movie. Please does anyone know how I can makethem stop this??? I dont know what to do!