1973 ford f-100. will not keep charge. New battery, new alternator, new voltage regulator?

Sounds like you might have a power drain when you truck is off like a light or other power source. Pull the positive cable and connect a test light from cable to post. If it lights up, pull fuses one at a time till test light goes out. That fuse is the system where the draw is coming from.

1. Most likely - It is possible you have a battery that will no longer hold a charge. If your alternator and regulator are working (that is, you have 12-15.5 volts at the battery while the engine is running), then your charging system is probably working OK & you may need to replace the battery. Parts stores can test your battery for you. 2. Least likely but I've seen it happen - In case your battery might just be OK, make sure you have nothing in your truck that is draining the battery. Turn your ignition switch to "off", then turn off all lights and accessories, then disconnect one battery terminal (it doesn't matter which) and connect a test light between the open battery terminal and the cable you just removed. If the tester lights up, you have something on that should not be. Double check everything is off - including the dome light if the door is open. If you have to, start pulling fuses until the light goes out. Once you figure out which fuse made the light turn off, check that circuit for possible bad components.

I'm not understanding the question. After you replaced the battery, alternator, and regulator it never started or it did but it won't now? If it was starting but the battery keeps going dead you have a short somewhere in the wiring.

If it never started you might need a starter or starter solenoid or even ignition switch.

I would have to say that your ignition module is your problem,you will find it under the hood on the inner fender ( drivers side )it's a small mayel box with wires pluged into it. All you have to do to remove ot is un screw the screws that's holding it down ,now that you have it un bolted turn it up side down , there's a clear coat like stuff under it and if it's been to hot the clear coat will be melted, you will have to replace it with one that has the same colour plug on the box.

Check all the cables for loose connections or corrosion. Those can cause a lot of funny issues and not a lot of people check for it.