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I want to make an auto transmission circuit using a magnetic pickup sensor with output 0-3.5 AC Volts.

My device has 3 gears (1, 2, 3) and the one that's activated depends on sensors output. For example, if the output is 1.5V, then the first relay is activated. If voltage is increased, the circuit should deactivate the first relay and activate the second one and so on.

How do I do that? Any ideas?

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The LM3914 is probably worth considering - it is a linear bargraph driver typically used as a sound level meter. It is able to drive one LED at once or be used in bar-graph mode. The point being that it has a bunch of analogue comparators and gates that virtually do what you want: -

enter image description here

Instead of LEDs you should be able to add a transistor to the outputs you want that can drive a relay (think ULN2003 for instance) such as here - it uses transistors to activate relays.

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