How to safely connect multiple switching power supplies to one single cold device socket?
Calculate the currents on the mains side using \$ P = VI \$ but solving for \$ I = \frac {P}{V} \$. We'll assume they're all 100% efficient.


The 300 W unit gives \$ I = \frac {300}{230} = 1.3\ \text A \$.
12 V, 20 A unit has \$ P = VI = 12 \times 20 = 240 \ \text W \$. \$ I = \frac {240}{230} = 1\ \text A \$ approx. 
12 V, 15 A unit has \$ P = VI = 12 \times 15 = 180 \ \text W \$. \$ I = \frac {180}{230} = 0.8\ \text A \$ approx. 


Even if your power supplies are only 75% efficient (resulting in 33% extra current on the mains side) you will draw about 4 A. You are fine.


  ... to a single cold device socket. (Whats the correct name in English for those!?)


Mains socket? Wall socket? 230 V power socket?

Make sure you have a fuse on the supply.