"My current strategy is to target the top 3 all on one page."
Definitely have one page for all those keywords. "Managed IT Services" and "IT Managed Services" are the same thing. Even "Managed IT Support" could be considered the same thing. Google's algorithm is smart enough to decipher synonyms.
Resource: http://helpmyseo.com/seo-tips/296-language-synonyms-and-the-google-algorithm-and-how-it-affects-your-seo.html

"The problem then is the title tag: Managed IT Services | IT Managed Services | Managed IT Support"
Definitely do NOT do this. Google will see it as keyword spam/stuffing and it will hurt you, not help you. I'd recommend making the title tag "Your Company Name | One of those keyword phrases" (assuming your company name doesn't already have those phrases in the name)
Resource: http://www.pagetraffic.com/guide/keyword-repetition.php

"I could build pages for all 3, but how would I incorporate them into the website since they are all synonyms. . . How would handle separate pages with synonymous content?"
The way to handle pages with to avoid them like the plague.
Resource: http://moz.com/blog/duplicate-content-in-a-post-panda-world

Why don't you try domain forwarding? That may be your only option as there is growing evidence that Google passes links from 302 redirects as well. You can find the domain forwarding option in the control panel where the domain name is hosted. Instead of doing a redirect, the change will take place with the DNS.

Here's some info: http://support.godaddy.com/help/article/422/forwarding-or-masking-your-domain-name

Make sure you choose forwarding without masking.

"If there is one different word in each description, that is presumably enough to stop them being seen as duplicates?"

Yes, if there's only one character different, they won't be seen as duplicates, at least by machines/bots.

"Also, I was intrigued that you said "Google doesn't officially use meta descriptions" - is there some doubt over this? I thought they'd confirmed it themselves?"

It's just that Google doesn't reveal exactly what they use in their algorithm to determine ranking, so its up to us to figure it out as best we can with observation and experimentation. Sometimes they will confirm or deny using a specific factor, but it's still up to the individual to choose to take that at face value or not.

"I think the choice I have to make is, is it better to have 100% unique descriptions that are too short (keyword only, as at present), or much less unique ones that are 130+ characters. I guess the latter wins."

Probably the latter, but in the case of meta descriptions, I would make that decision based to what is best for the user/customer rather than what we'd speculate Google would want.