If my unit deploys in mid-March, but I don't report until May, will I join them or be put on Rear Detachment?

If you are going to a BCT you are deploying.

Completely up to the unit commander. If they are short on bodies, you'll go. If they are close to their force cap, you'll stay.
I've been in bother situations. One deployment, I deployed a few months late and was stuck with a different battalion the entire deployment. Another deployment, I arrived just after the train up was completed and forced to stay on rear d because of a force cap.

What unit are you going to? Are you in a BCT? Like LTC (Join to see) said, if you are going to a BCT and your details there are accurate, then yes you should expect to deploy and meet up with them.

I just found out that I will be heading to 3r AR BDE instead of 2 IBCT which is the brigade that is deploying next year. Does anyone have any experience with 3rd brigade, what is the life like in the brigade I am sure I will live in the motor pool. I would preferred to live like a grunt for the next couple years but with that aside does anyone know if it's a good unit or if there is any deployments or will I be a slick leave for another 4 years.

If it was me, I would contact the unit (probably the S3 Operations Sergeant Major) and ask him for their intensions once you sign in. They can probably already see you on their gains roster and have already placed you within the command. What they may tell you and what actually happens are two different things, I know. I would want a good feeling at a minimum. If they were to tell me straight out that I was staying behind, I would look for a new unit. Possibly the next one in the chute to deploy. Or they may tell you that the BC already made the decision that all new members getting to the unit from D Day + what ever is deploying. You may fit into that equation and find yourself overseas. You need to be active in your placement or settle for what is given to you.

It is a numbers game most times.
If they are below deployed force cap plus know losses of those to be redeployed you will go once processed for deployment in most cases.

However some commanders take the long view as well if they see a soldier come to the unit after deployment that they believe will be a significant asset to rear D or that can set the unit up for success upon redeployment they may elect to leave you behind and be an asset at home

It can go both ways. But your probably going to be on rear D. Because 9/10 the person your replacing is on deployment with them. Once again it can go either way if NATO orders hit then you going across the pond

We had a young soldier on a mission once and he was standing at parade rest and saluting the LT. Turns out he'd just finished basic/AIT a couple weeks earlier and went straight to Taji. The BCT had already been downrange for over 6 months by that time. So yes, you may meet up with them.

cc: SGM Erik Marquez

If they aren't three months from redeploying, you'll probably go meet them.

It depends on the unit commander, but be prepared just in case you are selected to deploy.