3 hours once a month is nothing really. As many others have said, when they say they are visiting, you can say what time suit you. When my PIL say they are visiting, I usually ask them what (approx) time they'll be here, gives me enough time to wipe the jacks and hide the crap in the utility room, If I'm not available I tell them that and arrange another day.
Your dh is well able to help you, if he is not going to be there, arrange the visit for a day he will be there and if he is so useless in the Kitchen he can hold the toddler. I'm sure he can butter bread or stir a pot of soup.
They probably haven't invited anyone over because they are hoarders ?????
I get some people like invites, some just pop in, some like neon lights with fireworks and some people don't like visitors. I do honestly think though that for the sake of 3 hours once a month suck it up.
People change, my own parents worked full time, I rarely seen them when I had kids, then they retired and they wanted to vist alot, but then I didn't mind as I enjoy visitors and usually cook up a small banquet when people visit, unfortuantely their health go bad and both died before they could even settle into a visiting routine. Your PIL have more time now, they want to see you and their GC, they could be ignoring you completely and that would be worse.
If you are struggling to manage to cook meals, which is a BAD idea as you need to eat yourself and you need to have set meals for your child, you can buy larger high chairs, jesus some of them are like flying spaceships now with bells and whisltes on them, your ds would be delighted with himself and occupied by YOUR side while you make something to eat (not just for visitors) you can also get the standing yokes on amazon, I bought 2 of these, the kid stands inside what looks like a tall wooden box (sorry can't remember the name of it will look it up) but you can adjust the standing height, it lets them stand beside you at a safe height for when they are very active and want to be involved in everything, the stand goes right up against all counter tops.
What you could do and this is what I do, when we arrange a date for PIL to come over, they come over once a week but the day can differ, we usually arrange it begining of week, the day before I cook a lasagna or a shephards pie, we have that for our dinner and I make it bigger and use the rest the next day they call, 5 minutes in the microwave with supermarket salad bowl on the plate it's done and dusted. You'll be cooking dinner anyway the day before so no harm doing it a bit bigger bang whateven is left over in the fridge and bob's your uncle.