Is 3Mbps the speed you get by speedtest? How about the speed shown on the status of "wireless network connection"?
If yes, there is much difference in conception between the 3Mbps and 150Mbps.
3Mbps refers to the download speed of your internet; while 150Mbps refers to the speed of wireless connection; however, the speed of wireless connection you finally get is decided by your wireless card. For example, if the router is 150Mbps; while the wireless card is 54Mbps; so the real wireless connection speed is 54Mbps at most.

Kurtygus wrote

Model : TL-WR740N

Hardware Version : V4

running at 3Mbps when it should be running at ... it is quite frustrating trying to connect with such a slow connection it would be great if anyone could help me out

Very interesting - my father in law purchased this same router, and the best speedtest.net result he can get through the wireless is 1Mbps. The wireless connection shows 54M at best and often less even with the computer sitting three feet from the router.

The same computer connected wired to the router's LAN gives 50Mbps (router is fed with fiber to the house).

I updated the firmware to the V4 130529 and tried every combination of settings I could find on the net.

I'm starting to think the router is just junk with a worthless three year warranty.