My CO gave me a heads up that I'll be the acting company commander in March drill. What should I expect as a 2LT?

The AS3's aren't in the HHC chain of command. You are.

Get a copy of the unit training schedule and see what your company is up to. Review whatever metrics your unit uses for services, inventories, calibration, FLIPL's, and maintenance. Use the S1 through S4 officers and NCO's to ID deficiencies that can be fixed or at least be started to be fixed at drill weekend. That way you can answer any questions that arise from the battalion command staff without having to go searching too much. As an acting CO, there wont be much coming across your desk to do with command stuff, but your job as XO is always in the foreground and needing to be done.

Out of the frying pan and into the fire! Find the Sr NCO / acting 1SG and get some time with them ASAP. At an HHC, you are in a unique position where your Soldiers outrank you by pay grade, but you are responsible for them by position - be respectful, but firm that you are filling in and not making command decisions that extend beyond the BA weekend. Ensure you know the training schedule front and back and just keep the lights running! You got this.

I was there 45 years ago. I was a SSG, but none of the SFCs in the company, all coming up on retirement, wanted to be the acting 1SG when the 1SG was out for an extended medical issue. Fine, but you didn't get to be the SFC and ignore the 1SG. When I got pushed back by one of the Plt Sgts, I talked to the CO and told him that it was one way or the other. He agreed, and spoke to the three SFCs that didn't want the job (why that was allowed to happen is beyond me, but MI companies could be weird). From that point on, I was the 1SG, and there were no discussions about roles and responsibilities.

Likewise here, you're the acting CO. As others have pointed out here, talk to the other officers and NCOs and go for it. You don't have to be a moron about it, but you're the guy, so do the job. I think that LTC Trent Klug's advice is sound. Be sure that you know what the training schedule says, and follow it closely. Talk to the Bn staff as required. As the HHC CO, you will have people senior to you in the company, so thread the needle in terms of your role and your rank. Have fun!