I have a 3B in AA ball, who's a no-brainer for the big leagues eventually.  He's 20 years old.  
Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Chris Coles

He projects to this(current/projected):
Contact: 41/47
Power:  80/83
vsL:62/78
vsR: 69/77
Eye: 70/85

My current ML 3B is a free agent after this season and I'd love to sign him on for one more year, but he's asking for 3.  I won't need him in 3, as Coles will definitely be ready by then.  I'd love to keep Coles in AA this year and get him a full year in AAA next year prior to promoting him, but I think I'm not leaning towards promoting him to AAA for the second half this season and let him take over the 3B job for my ML team next year.

Will it stunt his development if I promote him to the ML too early?

Nope, as long as he gets his due playing time it wont matter

But there is always the chance that immaturity may cause actions unrelated to his on field play to disrupt the organization.

Your pitchers will hate you for letting that guy play 3B, but he should be fine at the plate in the majors.

It will just cost you more to promote him.  If you promote him before he's fully developed, you'll be paying for an average ML hitter instead of a very good one.  Also, his salary demands will come earlier in his career.  Add to that the fact that his fielding numbers are really RF-quality, and not 3B, you should consider hiring an FA to take over the 3B spot.

I agree with maxdeardorff...IF your set on your guy playing 3B I would keep him in AA this season, promote him to AAA next and pick up a free agent 3B to handle it for one year. By then your guy should be ready. A better option may be to sign your current 3B for the 3 years he's looking for and groom your prospect for the RF job, which is where he really should be anyway. Good luck either way!

I'd sign your current 3b to the 3 year deal with a mutual option for the 3rd year. Give coles another year in AA then the following year in AAA. Then if you still want him as a 3B then decline the option on your current guy and promote Coles. I'd really consider moving him to a COF spot

What defensive rating levels would you guys consider adequate at 3B?

In terms of players that I try not to go under the recommended mins, SS, C, CF, 3B, 2B in that order.

Take that for what it's worth.

Rarely have a 3B below mins, because that's where I stick my good bat, not quite defensively SS players.

70 across the board will do.   Higher is better but get that and you won't be disappointed with the results.

Ok.   I don't think you guys can see the projected ratings for Coles, but he's above 70 on all categories except range (and it's 68).  Obviously, he won't be near the ratings next year, but I think he'll be close enough to 70 on all of them t hat he won't just completely suck at 3B.

I'd be shocked if he glove or AA is anywhere near 70 next season.