For C-@ and C-^, often you can just type Control-2 and Control-6. For C-_, you may have to hold down the shift key, typing Control-Shift-Hyphen. C-@ can often be generated by typing Control-Space. C-@ is often called the NUL character, and has ASCII value 0. C-_ can often be generated by typing Control-7 or Control-/. C-? (aka DEL) may be generated by typing Shift-BackSpace or Control-BackSpace or a key labelled Delete or Del.
Try Control with all of the digits on your keyboard to see what gets generated.