First and foremost, any money you put in, you won't lose. Any money your company puts in, you will have to check what the vesting structure is. Typical examples are: 100% at 3 years (which means unless you work for 3 full years, the company gets the money back) or 20% every year for 5 years (which means at 1 year anniversary you now get 20% of what the company has put in, at 2 year anniversary 40%, up to at your 5 year anniversary, you get 100%).
Secondly, the terminology for 'match up to 100% of 3%' means that you can put in up to 3% of your salary, and if you do so, the company will match that dollar for dollar. If you put in less than 3%, the company will only match up to the amount you set aside. If you put in more than 3%, good for you for saving more, but the company won't be putting in any more.
Except in the most extreme circumstances, the advice here is to always contribute enough to get as full of a match from your employer as you can. Because the return is otherwise unbeatable. I.e. 100% match == your dollar immediately becomes two dollars, and you are not going to find a 100% return on anything outside of maybe a lottery ticket.
Now, combining those two answers above with your plan to return to grad school -- you may have one of those most extreme circumstances. That is, if you know for sure you are leaving before your employer contributions will vest, then maybe it is not worth contributing.
That said, there are a lot of people who were 'for sure' leaving their jobs back in, say Jan of this year, who are still working at that same company because pandemic came and jibbered up their plans and don't want to risk quitting now and finding a new job in this current market. I am not here to toss cold water on your grad school plans, but life changes. The school you want to do to may shut that program down. You may decide that this working life ain't so bad. Etc. Etc.
In short, even if you don't stay long enough to get the vesting, I'd most strongly recommend still contributing enough to get the full company match. Because if you do stay long enough and haven't been contributing, I think the regret from not getting that match will be great. And, as above, what you put in still stays. And getting into the habit of having savings taken from every paycheck is by far the easiest way to establish good savings habits. I.e. you don't learn to spend what you effectively don't have.
The Roth vs. traditional is not the easiest question to answer. If one's situation were to remain static over their entire life, then it would not matter. I.e. if one made the exact same salary, had the exact same deductions, and had the exact same tax brackets their entire life -- it does not matter if one pays taxes now (Roth) or later (traditional).
That said, the above is true for nobody, real life earning, deductions, tax brackets and so are always moving.
A reasonably decent assumption here is that since you plan to go to grad school, your future paychecks will probably be larger than your current ones, and hence you are likely to be in a higher tax bracket later. In this case, it makes a little more sense to do Roth now. I.e. the taxes you are going to pay now to put money in are likely to be relatively lower than the taxes would pay out later to withdraw. However, this is not a given, depending upon what your retirement plans are, and quite simply whatever Congress does in the future.
The ultimate hedge here would be to see if your plan can do both. I.e. if you choose to contribute say $200, can you put $100 into traditional and $100 into Roth, or whatever blend seems right to you.