Creating a flow to divide a day in several time slots, or scheduling app

Did you look at the solution provided here by @AVP?
Guess it does What you want,
Read it here:

And you can Start a 2nd flow from an Else …
And I guess I have here a Flow with 3 Time-slots (yes, that is the simple max) that starts a 2nd Flow in the Else like you ask…

And Yes a If statement in the Then selection is not possible, you should move then to the Conditions, that looks the same to me unles you want to Execute ActionA and If (SomeOtherCondition) Execute ActionB
but then the Start a Flow would work…
That (multiple flows) also is the way to differentiate for 3 timeslots, as with one If condition you cant have 3 outcomes… a Condition is simple true or false… But imho differentiating beween the 3 timeslots should be done with 2 casceded flows both with one condition:

Flow1 is started, And Time Before 8u00 Then Action1 Else Start Flow2
Flow2 is started And Time Before 16u00 Then Action2 Else Action3