multiple I-94 cards
 

I am a H1B worker. I have three I-94 cards since I changed my job twice. My question is whether or not I need to return all of my I-94 cards to airline agent or just the most up to date one, when I leave the country. thanks.

CBP in Charge, not airlines
 

I think you are confusing paper documentation for your legal status with airline paperwork. I-94 is a legal document which has nothing to do with airline agents (see this reference). It's endorsed by CBP agents upon your arrival/entry to the United States (source here).

If you have 3 I-94 cards, I assume you either have been in and out of the States 3 times or filed for extension before. I have had a handful of I-94 throughout the years and only the latest one is the "valid" one since that's the most recent legal documentation of my entry. I don't recall I have any "old" I-94 with me since it's always collected by the CBP upon my exit (I think). Not sure how you get to hang on to 3 cards.

I am on student visa. I don't know if the rules would be different to work visa holders. I would think it should be the same though. I mean the most recent one is the "official" one that counts.

You probably mean the I-94 departure record. This is collected by the airline check-in agent for the CBP when you exit the United States. Forget to return the I-94 departure record can cause problems on you next entry in the United States but normally it doesn't. If you are afraid your entry could be denied follow these instructions: How to return i-94 departure record

I am curious to know that why you don't want to submit all 3 i-94s ? Is there any downside to it ? I am on H1 and I always submit my i-94s, sometimes even when airlines checkin agent forget to take it, i remind them. I don't want to have any trouble later on.

If it not a problem and there is no reason, then submit all 3.

If it's a one time I-94 then the GA will confiscate it when you board your flight back to your country.

If it's a multiple entry I-94 (stamped "Multiple Entry") then you just relinquish it to INS when you re-enter the states after you've finished your work or it has expired.

This is at least from my experiences in the past. Maybe things have changed now.