1996 Chevy Truck hard to start some times

Sometimes when I try to start my truck it starts hard, but then sometimes its fine. It tunn over hard like a dead battery the starter is brand new and so it the battery so its not the starter or battery. The fuel injectors are two years old and the reading that I get from the computor says catalists under efficiency. But then sometimes it says missfire on number 3 cyl. What would make it hard to crank over sometimes? The missfire on number 3 only come when it hard to start but then I do get it to start thank you