The one thing Urlacher has on both of them is athleticism, neither Singletary nor Butkus could do the things that Brian is capable of, he is a freak of nature. Having said that Brian still has something to prove, if he closes out his career with a ring, 4-6 more outstanding seasons of play and a hall of fame induction I don't see why he couldn't be considered the best of the 3 since Brian's numbers are certainly more spectacular than Singletary and the stats for Butkus are incomplete considering the era he played in.
As for the intimidation factor that some use for Butkus and to a lesser extent Singletary and against Urlacher you have to keep in mind that a lot of the things Butkus was allowed to do in his day (wacking QBs upside the head, twirling opponents in the air by their facemasks, etc.) Urlacher would be flagged, ejected, fined and suspended for. The new rules have taken a lot of the intimidation factor away from the defense but you have to believe that if these rules were eliminated today Urlacher would do some BAD things to people.