I have a Baldor DG6E generator. It must produce 120V/240V but I bought it and it has three wires as outlets - every one produces 120V. I want to combine two wires to produce 240V. How can I combine two wires to produce 240V or is there is another way to make the generator produce 120V/240V?
If the generator does not have a 240 VAC output then it's unlikely you can combine two 120 VAC outputs to get 240 VAC. The only way I can think of that is both safe and reasonably inexpensive (for some definitions of "reasonably") is a transformer.
How much power do you need from this 240 VAC output? If it's less than 1000 watts, or perhaps 2000 watts, then you could possibly pick up a step up transformer from a hardware store for under $100. Any bigger than that and it's probably cheaper to buy a new generator with a proper 240 VAC output.
An example of such a transformer here: https://www.homedepot.com/p/Power-Bright-2000-Watts-Step-Up-Down-Converter-110-120-Volt-220-240-Volt-Voltage-Transformer-VC2000W/302542246
The three wires are for Neutral, and two 120VAC lines which are 180 \$ ^\circ \$ out of phase.
The DG6E has a 120/240 VAC output using a NEMA L14-30R connector according to the manual.
You can get 240 VAC between the two 120VAC lines(marked Gold).
Check out the manual : https://www.baldor.com/Shared/manuals/2414-104.pdf How to use the connector : http://www.generatorsforhomeuse.us/l14-30/
As always, take extreme precaution when working with these and check the warnings.