First, trust you've read the relevant FAQs:
https://www.kliaekspres.com/faqs/

You're supposed to check in bags at KL Sentral >2hrs before your flight. I've never used the service myself (I don't travel 'heavy' through KL, so manage my own bags), but since you're looking at >3hrs and if it is a hassle managing your baggage, go for it.

However, business class check in at KLIA will be quite fast too (at least 1hr before flight). If you're rushing for a train (which you shouldn't need to w/ 3hrs) or if you want that extra sense of security, you just take the bags personally to the airport. I've never been on a KLIA Ekspres which is full, so usually there's place for one's allowed 3x30kg bags. (The 'local' KLIA Transit trains through Putrajaya can be packed though... but you would just go on the express KLIA Ekspres version.)

The KLIA Express is <RM50 if you tap in directly at the turnstile with a Visa *Paywave* card (+ foreign transaction fees) or RM55 at the ticket window just before, and will take <40mins to KLIA. It drops you off right below the terminal, so minimal walking too: essentially, you can just take the elevator directly to the top floor, and walk straight through the check in counters, get scanned by the guard at the escalators down to emigration (can also use OLCI / Passbook boarding pass too).

With a business class ticket, you can use the VIP emigration desk, and should be through a lot quicker even if there is a crowd; you may hit the crowd at the not-airplane-security metal detectors just after emigration, but usually those move quickly, and IIRC there's a Business class line to the left.

I would think that, from getting off the train to the main terminal airside, it would take no more than 20 minutes for a regular traveller in business class, so if one timed it perfectly, 1hr from Sentral to the Main terminal; for someone who is used o it, a train leaving Sentral 1.5hrs before a flight is doable.

Havig said that, as a first time traveller, take it easy. Leaving the Hilton at 10:45am to even get an 11:20am (or 11:40am) train should work out just fine.