Volts x amps = watts. So watts divided by volts = amps. On a 6V tractor, you say you have three 25-watt bulbs. Now, 3 x 25 = 75...and 75 divided by 6 = 12.5 amps, which is what your total draw from the lights SHOULD be. Divide that 12.5 amps by 3 lights, and each light SHOULD be drawing just under 4.2 amps. But if you've got a 20-amp draw, that's about a 6.7-amp draw per light, on the average.
What would I do in your position? I'd check the light circuit with a volt-ohm meter. With the light switch on, I'd check output voltage at the switch. If it's at 6V or slightly above, that's where it should be. If it's lower, then I'd check the voltage coming in to the switch. If the input voltage is at 6V or so, but the output voltage is less, then you have resistance in the switch, and you should probably replace the switch.
If the output voltage at the switch is in the correct range, then I'd check the voltage at each of the lights. If there's a voltage drop between the switch and the light(s), then there's unnecessary resistance in the wiring...and I'd replace the light wiring with a new wire, with soldered terminals, and with heat shrink tubing over every connection.
If the voltage goint TO the lights shows no significant voltage drop, then I'd check the bulb sockets to see if there's a voltage drop there. If there's no significant voltage drop anywhere between the switch and the sockets, then the excessive amp draw has to be coming from the bulbs themselves...and I'd replace the bulbs.
What you do is up to you, and I'm sure there are experts out there who would disagree with my methods. But that's just what I'd do.
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