Making a bistable or flip flop is easy . How to build a Tristable? preferably from three logic gates?

You need to define the rules. A divide by 3 circuit can be made from a few FFs, depending on conditions, if a divide by 3 is what you want.

A SR FF has a set of rules.

S R Q Qbar

1 0 1 0

0 1 0 1

0 0 keeps previous state

1 1 indeterminate

what are the co-responding rules for your tristable FF ?

edit:

so the logic would be

inputs outputs

R G O R G O

0 0 1 .. 0 0 1

0 1 0 .. 0 1 0

1 0 0 .. 1 0 0

0 0 0 .. keeps previous state

for the other states, I'd say off predominates over the other two

x x 1 .. 0 0 1 (x = don't care)

This can be implemented via two FFs or via two or 3 relays.

With the two FFs, the Red and Green inputs go to the set inputs of each of the two FFs

The Off input goes to the Reset or clear inputs of the two FF, in parallel.

So the permissible states are:

red on (green off)

green on (red off)

both off

You also need a power clear circuit to force the FFs into the clear state when power is first turned on. Usually consists of a large cap to ground and a resistor to +5v. When the power first turns on, the cap stays near ground for a fraction of a second. This is fed through an OR or NOR gate into the reset/clear inputs of all FFs.

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