Just because you have money doesn't mean you have to spend it.
I'm not opposed to any of these guys, but I don't necessarily want to spend the max for them. But all 3 have their upsides.
Of the three, I like the way Harris fits on the Celtics. He has his issues defensively but the Celtics really, really need a 3 - Crowder and Turner aren't long-term starters - and Harris fits nicely. He's good offensively and can physically abuse smaller SFs. Bradley and Smart can help pick up any defensive slack if he's over matched against a very quick SF (which is the kind of player who can give him trouble). Plus, I think he'd be the cheapest of the 3 and that's always a plus.
Middleton could also work with the Celtics. His ability to lock down the SF and the SG spots would make the Cs defense even more potent. He's not a 3 point threat (not really), but he can score enough that you have to respect him. I'd rather pair him with a scoring SG. Note that last year he spent a lot of his time at PF and I don't want him there. I want him at SF. So you evaluate him at that position - can he play it and play it well? Yeah, but I'm not sure he's the guy we need there.
Monroe. Oh, Greg Monroe. He'll likely be the most expensive and the riskiest of the 3 choices. He's going to get the max. The question is can he earn it. He's ponderous down the floor with exceptionally limited range and mobility. But he can clog up the middle. Here's my problem with Monroe - his stats EVER SINCE HE CAME INTO THE LEAGUE are basically flat. There's no growth in his game. He was a 16/10 player his 2nd year. He's a 16/10 player now after his 5th year. You're getting him hoping he'll be a 20/10 player and adequate on defense but I don't think there's any logical reason to think he will be. I don't really like the pairing with him and Sully either - it'll take 15 of the 24 seconds of the shot clock for these two to lumber down the court.