First bike for a near three year old?.?

I'd go w/ the one that has training wheels so you can just take them off when he is ready.  That way you only need to purchase a single set of aero wheels and bars. 

Seriously, just keep it simple.  There's nothing like that first little push after the training wheels come off, you look down and see that big smile on your kid's face and suddenly he just leaves you in the dust while you try in vain to run alongside.  You watch the end of the parking lot approaching fast, with the curb and chain link fence directly ahead, all the while screaming "SLOOOOW DOWN, TUUUUURRRRNNNNN !!!.  Your mind flashing with visions of your wife beating you with a rolling pin for having let your son crash hard after promising not to let go of him.  Not that I've ever experienced this.  Trust me, it'll all be fiiine.

This. 

We just bought the 12" trek for our 3 yo daughter.  She is having a blast on it.  She did spend all last summer on a strider (even did an Ironkids triathlon on it.)  I would say at this point there is no reason to buy a strider. Although I am a HUGE strider fan and we loved ours- took DD mountain biking with us and to the pump track.  She had to much fun.  (Heck, we bought her a Y-Bike for her first birthday when she wasn't ready for the strider yet.)  The good news if you do decide on a strider is they hold value really well in the re-sale market.  (Well, at least in Boulder :-)  We only bought the trek b/c she was begging for a bike with pedals.

I'd get the trek- but don't do training wheels- like the other poster said just take off the pedal parts and let him use it like a strider.  I'm pretty sure our daughter is ready to learn pedals if I just put the time aside for a weekend- the guy who sold us the trek gave us drills to practice with her and thought she would be ready after a few days of practicing the new height.  Watching friends kids the transition from strider type bike to pedals seems a lot easier than the removal of training wheels.

That said I can barely keep up with her on her strider jogging- so I'm worried she'll go to fast for me with pedals.

ETA- this is all based on your son being able to comfortably stride on the 12" bike and will depend on height.  If he's not tall enough for it definitely get the strider.

Wait.....what?  Your THREE year old did an Ironkids race? Where?

 When my kids were three they were eating dirt.

I think only in Boulder

She was actually two, but USAT age was 3.

It was a total blast.  She has done a ton of 5k/10ks in a stroller with us and watched us race at the res a lot.  So when I went to packet pick up for the 70.3 she wanted to sign up so I signed her up -it's on Sat before the 70.3.  It was not timed at all- just for fun for 3-5 yos.  25 yards running through the water (the lifeguards were shoulder to shoulder in knee deep water watching them), a 1/4 mile bike (she used her strider and mom ran with her) and a 250 yard run (with mom as well.)

She still wears her finisher medal all the time and keeps asking when she can do it again.  Doing the race with her is one of my top memories of the entire year last year.

Here is a picture of the transition area for the 3-5s at Boulder Ironkids- not super clear- but you can see there were all types of bikes.  Striders, training wheels, two wheelers.  Kids love any kind of bike.

http://www2.snapfish.com/snapfish/slideshow/AlbumID=10055791009/PictureID=557810523009/a=25836345_25836345/otsc=SHR/otsi=SPIClink/COBRAND_NAME=snapfish/

and happy finishers

http://www2.snapfish.com/snapfish/slideshow/AlbumID=10055791009/PictureID=557810525009/a=25836345_25836345/otsc=SHR/otsi=SPIClink/COBRAND_NAME=snapfish/

Awesome stuff.

12" and remove the pedals.  

My 7 year old finally learned to ride this past summer on a 16" Trek.  Used to ride a 12" with training wheels when she was 3-5 then we moved houses and she quit riding.  Finally talked her into learning on the 16" this past summer.  Two weeks after she learned, she entered her first kids tri (50m/2k/1k).  I ran the bike course on the sidewalk as I was a bit nervous with her just learning how to ride without training wheels.  She had a blast and I think I had more fun watching her than any of the races I've done myself.

Less than two weeks later, my 5 year old learned to ride without training wheels on a 12".  Anything big sister can do, she can do.  Both are planning to race a couple kids tris this summer.

That is awesome (and cute)! This thread was timely. Recent conversation with my daughter who just turned 3, a few weeks ago:

Katie: "I want a bike when I am a little bit older"

Me: "How much older?"

Katie: "When I turn 4"

Me: "What color should it be?"

Katie: "Pink"