For your formula, you can remove IF() since the visibility formula will already check if the formula will return true. Also, I see an extra "Onboarding_Plans" in your formula; perhaps that was an accidental auto-complete when you were creating your formula.
This is how your formula would look instead:
ROWS(FILTER(Onboarding_Plans,"Onboarding_Plans[name]=% AND Onboarding_Plans[dob]=% AND Onboarding_Plans[Seniority Level]=%",$[SYS_USER],"", "Contractor"))>0
Hope that helps clarify, and let us know if you have any other questions!
Also, thanks for chiming in with a solution, @DannyHoney! Using AND as you've shown is another way to write a visibility formula that would evaluate to true or false.