I think Boldin is better than all three of those guys.
If i'm starting a team, i'd take Boldin over any of them.
Johnson is a one dimentional deep threat who's career declined rapidly, like most deep threats do.
TO destroys teams and is worthless over the middle with questionable hands.
Marshall is another guy who has questionable hands, and has only produced for a short period of time.
Boldin has glue sticks for hands, is tough as nails, has great size, can run any route effectively, plays hurt...the guy has it all. He's a much more complete player than tthe other three.