How lousy of your daughter to put you in this position! So she had two kids, couldn't afford childcare and asked you for help, then managed to have another? I'm sure you love her and your grandchildren, but that's infuriating.
She's taking advantage of you. Rather than give in when she throws a tantrum about boo hoo, poor her, she can't afford this, she'll have to move, blah blah blah stay calm and respond with something like "I hear you, but it sounds like you're not hearing me. I cannot do this. So what's your plan? Let's figure out next steps."
If she has a partner in her life, then they need to grow up and figure this out. If she is single and low income, she needs to research what kind of aid she can qualify for, if any. And needs to get child support from the children's father(s) and put that toward childcare. Or move to someplace more affordable. You've raised your child(ren) - she's responsible for raising hers.
Are you in a position to help out a few days a week instead of full time? Maybe you have the kids on Tuesdays and Thursdays and she pays for childcare on Monday, Wednesday and Friday? Or are you financially in a position to assist with paid childcare if she can't get a voucher and can't afford full time care? She should be able to use a before and after care program for the school-age kids and really only have to pay for full-time care for the younger child. It's not like she needs to pay for full time care for three kids.
FWIW, I live in an area where the cost of living is ridiculous and full-time childcare is $1500+ per month per child and the average family pays almost $30k a year for childcare. There was no way I could afford that (even when I was married) so my parents helped out a few days a week and it was a nice compromise. I saved money, and they got to spend time with their grandchildren but not all day, every day.