It appears as though something has been garbled. USt 3 is a designation for a low carbon steel in the now obsolete German standard DIN 1624 Flat products of steel; cold rolled narrow strip in widths < 650 mm of mild unalloyed steels; technical delivery conditions. There are other descriptors in this standard such as G and LG used to indicate that the steel is annealed (G = geglüht) or temper rolled (LG = leicht nachgewalzt). A typical designation should appear as follows:
Stahl DIN 1624 - USt 3 LG BK UG FE
LG means temper rolled
BK means bright surface (Blanke, metallisch reine Oberfläche. Poren, kleine Narben und leichte Kratzer sind zulässig.)
UG means electroplating quality (vorgesehen für elektrolytisches Oberflächenveredeln)
FE means degreased surface (mit entfetteter Oberfläche)