A 2D file that was made to look 3D… often a problem. The ‘C’ is different from all the other letters (a chiselled bevel), it could be done (almost) with an extrude but the spline would have to trace the flat part of the C and the ‘back of the C’ (on the left) would have to have the spline touching itself for that part – then the extrude would work and create the outside – however getting a spline to align with itself like that is tough (not impossible though).
The other letters can’t be done the same way since they look like they are done in a different 2D technique the “3D fake drop shadow” kind of thing. Because it’s asymmetrical you can’t just use an extrude (unless you tilt them to the camera) you’d have to make 2 splines for each letter one for the flat ‘face’ and another for the outside shape. If you then put the front spline forward and the outside spline back then join the two with a Loft you’d have the letter. — just had a thought, maybe an extrude could work if the extrude was made to go “not straight back”, as is usual, to end up with faces that face the camera but sides and back that have the needed “drop shadow” alignment – just tried that and it doesn’t work well if the fillet is too large and it also suffers badly from true 3D – it’s obvious the letters (the drop shadow effect) are all a bit different because they are all in different positions relative to the camera, the letters in the example jpeg all have an identical 3D effect, it is possible to do that in 3D but you have to change from the perspective to the Parallel camera to do it.