Well. what else could they say. Ruby stalking Oswald is proof enough, but to each their own. 
The Three Stooges have no plan - or at least no effective plan - to eliminate Oswald before he escapes the TSBD. The Three Stooges have no plan - or at least no effective plan - to eliminate him before he hops a bus, hails a taxi, and enters and leaves his rooming house. The Three Stooges have no plan - or at least no effective plan - to eliminate him as he walks down the sidewalk. (If Tippit was the plan, that was unworthy even of the Three Stooges unless Tippit was a Fourth Stooge himself.) He isn't eliminated when he flees to the theater, resists arrest and pulls a gun.
"Dang, what are we to do now?" asks Curly, but Larry and Moe just shrug. Wait, says Moe as a 20W light bulb goes off inside his head, we'll hope he doesn't crack while in custody, recruit a low-level thug with all sorts of red flag underworld connections to stalk and kill him, and hope it somehow all works out. We'll either threaten to kill the thug's entire family or promise him $100K or maybe both.
"Genius!" agree Curly and Larry. Ergo, Ruby prowls around police headquarters with a gun in his pocket, looking for his chance. It finally comes in a crowded basement full of reporters and TV cameras as Oswald is transferred long after he was supposed to have been. As part of the plan, Ruby stands in line at the Western Union office until the last minute. "No risk there," says Moe. "It's perfect!"
The deed is finally done, and the Three Stooges breathe a collective sigh of relief until Curly, the brains of the outfit, says "Wait a minute - doesn't Ruby now pose as much of a risk to our operation as Oswald did?" Well, yes, his brothers agree, so now we'll have to hope Ruby doesn't squawk before we can, oh, inject him with cancer cells or something like that. "It'll all work out," Moe assures them.