Will the 3-4-5 method work for making a table support?

Instead of making a table support leg drop down vertically to the floor I wanted to make an angle (45 deg) support. If I measure "A" from the floor to the table top and measure from wall to the end of table top...

Can I assume that "C" will correctly fit with a 45 deg angle at both ends?

NOTE: I could not get my graphic to make a 45, but I meant a 45 deg cut at both ends of "C".

No, 3-4-5 creates a right angle, not a 45 degree angle.

In your example, “a” must equal “b” for “c” to have 45 degree angle cuts at the ends.

Yes, you can assume that it will fit, but that won't get you a right angle in your table. Just cut a brace with 45 degree angles at each end and be sure that the ends are equidistant from the inside corner formed by the top and back panel.

If you want to know the length of your brace, decide how far down/out you'd like it, then use the usual formula:

length = square root ( 2 x (your distance²) )

So, for example if you want the long point of your brace 14 inches out:

brace length = square root (14² + 14²)
= square root (196 + 196)
= square root (392)
= 19.8"

If you did want to use the 3:4:5 method, the miters on your brace will need to be approximately 36.9° and 53.1°.