Synchronizing Lamps

I do not understand your question. How does the generator and lamp relate?

the 3 lamp used for synchronizing,(manual synchronization)

Well, what is the maximum voltage between phases? Oooh look, a question answered with a question. Tutty-tut.

120 VAC since you will always have a ??? to 120 volt PT on your generators output. I'd use a synchroscope instead of lamps, then you can ask your Electricians where the 2 phase motor is in the plant

Synchronising lamps are USUALLY connected to the machines by transformers, but can be directly connected. The essential point is that a synchronizing lamp sees zero volts when the generators are in phase but Two times the generator voltage (or its transformed value) when 180 degrees out of phase. Thus if you have 220 volt single phase generators and connect lamps direct to them the bulb will see 440 volts. There are not many 440 volt panel lamps made! This is one reason Sync lamps are usually fed via transformers. For direct connection, you would have to add a ballast resistor in series with each 24/50/120/220 lamp according to what standard voltage lamp you use. Standard secondary voltages for voltage [potential] transformers used on Public Supply electrical power gear are 100, 110 or 120 volts when the high voltage side is at its rated value. It is according to whether you have Continental Europe, British or USA influenced gear respectively. If your synchronizing lamp transformer has a secondary voltage of 12 when the machine is running, you will need 24 volt lamps. The wattage of the lamps will depend mostly on the dimensions of a lamp which will fit the lamp housing/socket, but avoid lamps which leave very little space for air to circulate in the lens or have a high wattage for their size.

Sorry, I was wrong in the "last but one" sentence of post #5. Maybe you have the world's most basic system? - as in diagram below.

Here lamp L1 shows that generator G1 is "live" and L2 shows that G2 is live. L1 and L2 must be 220V. Lamp L3 will be dark when G1 and G2 are "in phase", but when G1 and G2 are 180 degrees out of phase, L3 will see 440V. Thus L3 must be a 440V lamp or two 220V lamps in series or a lamp < 440V with a resistor ballast or transformer for 440V. The lamps can be fed by identical transformers, having secondaries commoned together at one end, a lamp L3 between the other ends of the secondaries and lamps L1 and L2 across secondaries for G1 and G2. The lamps will function the same as in the diagram [and still need L3 to have twice the voltage rating of L1/L2].