Custody swap is killing my schedule - need real talk

So I have my kids (7, 9, 12) Mon-Wed and every other weekend. The problem is my ex keeps pushing swaps last minute because of her work stuff. Last week she asked me to take them on my off day THREE times. I said yes twice because I didn't want to be "that dad" but now I feel like I'm being taken advantage of.

How do you guys handle this? Do you just say no? I genuinely don't mind the extra time with my kids but I'm losing my other job flexibility and she's not reciprocating. We tried using OurFamilyWizard but she barely uses it - just texts me randomly.