There’s going to be times where you can’t be in two places at once like birthday parties, activities, school events ect. I’m sure the grandparents will enjoy watching him and he’ll understand. Although two days seems optimistic!
You support your oldest every weekend at basketball. Look at it as supporting your 3yo this weekend doing something that is important for them too. Feeling guilty only shows you care about your oldest. Make sure you tell him how proud of him you are and how much you will miss seeing him play and how excited you are for the next time.
Don't feel guilty. I have three and you can't always be there for everyone all the time. My oldest does sports and we typically just try to make sure someone is there. A parent, a grandparent. It's just not always possible for all of us to pack up and go somewhere.