Well, as a couple, what do you AND your Husband think?
Are your in-laws basically respectful normal and pleasant people who respect you as parents and respect your kids and don't go around undermining you and nor talk badly to your kids etc. and know about child safety and can properly... supervise... THREE kids at one time? And they are able bodied and are safe?
And how old are your kids?
And, it is up to you, IF you want them to babysit or not or have your kids over, or have them over or have them tag-along with you as a family.... per anything you do or outings.
Or, you can opt to plan... outings, in which your in-laws can tag along on, too. For example: we are going hiking with the kids, do you want to come? Or, I am making a pot roast, do you want to come over and eat dinner too? And YOU state, the timing to things, and the start and finish times etc. So that, "you" feel in control of it per your own family's routines... versus your in-laws... laying down the schedule, on you or your kids.
And just communicate diplomatically. ie: " you wish to see us one night a week, we have 3 kids and with all their activities/homework obligations, Sunday afternoons are the best for us..." and then let them reply with their reply to your parameters.
That way, you are considering their 1-night a week 'request' but yet, you are also diplomatically, stating your own family's schedule etc. to them.
But it does not have to be, a stringent all the time, thing. I mean, they only live 20 minutes away.
And you do not have to... let them or have them, babysit once a week. And it is up to you, if you even want to leave your 3 kids there with them.
They wish to see you, the kids, once a week, at night. So when... might that be.... ? Instead of letting them decide the night/date, you decide.
And if some weeks you are all just too busy, then say so.
It is normal life with kids.
My kids have an Aunty who has no kids. SHE used to think... that anytime she said so, she could see the kids or come over or whatever. And she'd get irked/mad... if we were busy or were not home or were not home when she... called on us, or if my kids were NAPPING when she wanted to see them. Then, as my kids got older, it got more busy and my kids have a social life too and invitations and/or play dates etc. Such is life with kids and a family. Well, so I had to pretty much... explain to their Aunty, that, it cannot just be spur of the moment. And we cannot just stop, because she spur of the moment wants to come over or take the kids. And it is up to us. We are NOT being rude, by saying so... we are just giving her a head's up on what to CONSIDER... when she gets in the mood to get together. Because, she is a persnickety picky person... who has her own ways.
Then, I have a friend... who has 3 kids. AND, well... all the time... their Grandma and Aunty are with them. Always. Everywhere they go. Everywhere... literally. They have, no privacy or 'free time' just to themselves or with their own kids. And the Husband gets tired of it. I mean, his in-laws are always...there at their home, or tagging along to wherever they go with their kids. And it is... too much.
It is just, too much. And yes, dreaded. Because it is an obligation at that point and if they stop, the in-laws will get mad.
The thing is, once a person has kids... you either are okay about the in-laws/relatives always having your kids and asking for your kids... or not.
Because, it is not a have to, thing. You are the parents.
I see some parents, that are always sending their kids to the grandparents... because they think they HAVE to...as though the Grandparents have more 'claim' on the kids. Or other parents that don't... because then the kids are hardly with the parents themselves and are always at the grandparents.
But in your case, it seems perhaps, your In-Laws simply want to see your kids, because they hardly get to, even if they live so close by?
And if you agree to the once a week thing, then you/Husband can say... diplomatically, what the best day/schedule is for it... and the timing of it. ie: do you want them over all day and night? Or for maybe 3 hours in the later afternoon to evening?
Just say so.
And if once a week it just no doable... then say so. Then offer up an alternate day that will work.
I think, from what you say... you feel "obligated" and tied-down... about it, because the In-Laws are requesting "one night a week...." type thing.
And it seems so set in stone.
But it does not have to be.
Just say when it is, doable. And that it does not HAVE to be, one night a week... forever.
As parents, you explain nicely to them, how busy it is... and when the best time would be.... then you have the handle on it and won't feel like you are controlled by them or at their beck and call. And it is up to you if you get together all of you, or just the kids to them or they come to you.
Currently, do they not see you all... already? Or hardly?
You do not have to let them babysit. Or you can.
It is up to you. And how often, if any.