Hi,
A little bit more infomation and we might be able to help you. What car are you driving/or is it any car?
Maybe its just the pitlane speed limiter? Press "L" to turn if on/off. The limiter only restricts the speed to 50 mph / 80 kmph, but you'll be able to shift up.. So
Join an online race and do a lap or something until we can see that you cant shift upto 3rd. Then save the replay (press 2 - and give it name). It'll be saved in the LFS/Data/mpr folder - upload the .mpr file in your next post using the attachments {} button and someone will be able to say what the problem is...
PS: What type of controller are you using? Wheel/kb/mouse/xbox controller? Is this the first time the problem came up since you got LFS?