"Two galaxies traveling at 3/4 light speed in opposite directions. Einstein proposed a scenario composed of two galaxies moving at 3/4 light speed from the earth, traveling in opposite directions. This meant that they were traveling away from each other at 1.5 times the speed of light. He suggested that a message sent by one to Earth and relayed would eventually reach the other. Since this meant going faster than light speed, how was this possible?"

Although he uses relativity as his explanation, I will not. When the message first leaves the original galaxy, is it traveling at full light speed, (not light speed minus 3/4 of light speed). From the location that the message was sent, regardless of any extraneous Earth relay, it will overtake the intended galaxy at a constant rate of 1/4 light speed. This begins from the moment the initial message was sent.