List validation - Multiple Choice Field --> some choices make another column required

Question

There is a list. One column has 6 project states. Another column is for when the project will start. We want to connect the 2 when say 3 of those states are chosen(but continue to not be required if the other 3 are). So if Option1, Option2, or Option3 are chosen, then make date column required.

Going through guides, everything is for Boolean choices, and I can't seem to find the right method, nor do I understand the formulas very well to begin with. This is SP2016 and using List Validation.

=IF[Phase]=Option1,Option2,Option3THEN[Date]="True" ?? (I stress this is not my strong point)

Answer 1

Try this:

=IF(AND(OR(OR(ProjectStates="Option1",ProjectStates="Option2"),ProjectStates="Option3"),ISBLANK(StartDate)),FALSE,TRUE)