I'll consider the water tank as an equipment load. A concentrate live load, or an uniform live load in a specific area of the roof.
If the location of the tank is unknown at the time of design, you have to make sure anywhere around the roof can take this load, on top of typical roof live load.

I have many water tanks(unkown number) of 3000L with unkown loacations how can I apply their live load?
Also is there any maximum size of tanks to be able to consider live load of roof given in ASCE sufficient?

For everything unknown, I'll use the water tank weight as live load through out the roof. Use the largest tank possible.

Water generally weights 62.4pcf. So if your roof were a bowl (think parapets with a waterproof liner), a roof designed to ASCE 7 for typical roof live load could hold...0.321 feet of water...or 3.84 inches. Not much. It will depend on the shape of the tanks. If they're wide and short and bearing directly on the roof, it'll be spread out more. If they're tall and skinny, you could have an extremely high load. If they're somewhere between the two but sitting on a frame or some sort of grillage, you could be looking at some significant point loads.

In short - the 20psf live load is NOT for water tanks. A water tank is a unique applied load and should be treated carefully. Ask questions, find out what the actual load from the tanks are, and how many you'll have.

Agreed, 20psf live load and tank load need to be separate (and in many cases, additive... you should be able to have workers perform maintenance on the water tanks).

In my area, cylindrical metal or plastic tanks are common, which do sit on a frame. The point loads are important. Most frames have at least 4 legs, so you could consider 7.5kN or 1.7kips (unfactored) at each leg. The spacing of those legs will depend on whether your tanks are horizontal or vertical -- but they're often around 0.8m.

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just call me Lo.