How to use 3 gear bike ?

Check your compression on each cylinder. It sounds like you have holed a piston or lost a piston ring.

I don,t think the chain could cause this , try lubing the chain with oil and taking it for a ride see if that makes a difference , when was your fuel filter last changed or your air filter cleaned or changed.

Sue, there are lots of how-to videos out there about this. Just google "How to change bike tube" or tire, or flat. You have to remove the wheel, which is easiest with TWO wrenches - they can be the adjustable "Crescent" wrenches, but don't use pliers. You also have to remove the small screw that holds the fat little L-shaped lever to bike. Use a rag to remove the chain from the cog on the wheel. Then use the little "Tire irons" to remove the tire from the rim and get the old tube out - these come three to a set, the plastic ones are best, and you can get them at K-mart. Also, use some baby powder (talc is better than cornstarch) to make the new tube easier to position correctly in the tire - sprinkle the powder inside the tire when you have if off and put the tube in the tire before you put the tire back on the rim. It helps if you put a little air in the tube to make it a nice sausage first. And make sure you get the valve stem straight and pulled all of the way through the hole in the rim. Be very careful that you don't pinch the tube against the rim when you put the tire back on. And the hardest part might be getting a pump to hold tight onto the valve stem - using compressed air (like at a gas station) is by far the easiest method. Good luck.

A fixed gear bicycle is a single-speed machine with a fixed drivetrain, meaning the pedals move at the same rate as the rear wheel. It is impossible to coast on a fixed gear bike, leading to a different style of riding from traditional free wheel-equipped bikes. Additionally, many fixies do not come with brakes, requiring you to learn how to stop the bike through other methods.
Although it is possible to ride a fixie without a break, it is very dangerous to do so on the road. Install a front brake so you can stop short in an emergency. Make sure your frame uses horizontal dropouts.
More details about the Fixed Gear Bikes open the link
Best Fixed Gear Bikes In 2018 Enjoy Your Ride On Cool Fixie Bikes

as far as I know they aren't meant to start in gear as a safety feature so people don't start their bike and it immediately takes off on them

Look online. Web search "how to shift gears" or whatever other questions you can think of and you will probably find all kinds of resources and videos. Don't get TOO specific as there are are only so many variations on the standardized mechanisms hung on your bicycle frame.

Why don't you just use a search engine for either request. Giant has generic manuals for its bikes and the web has videos for how to shift. The answers would be faster.

Bikes is bikes so ANY owers manual would ge tyou through the basics.

TRy Huffy's. http://www.huffy.com/Support/OwnersManuals.aspx

Then, using a search engine, ask any question like "how to shift a bicycle" and videos will appear instantly.

This doecument will teach you a whole bunch of stuff about techniques and strategy for road riding.

http://www.johnforester.com/BTEO/ECIL.pdf

The bike has a slightly bent shift fork. The repair is a pretty major operation and I don't recommend the casual mechanic attempt it. Below is a diagram showing how the shift drum moves slider gears for different gear changes. Please rate my answer. Thanks.

so did you just get new boots and had to adjust the shift lever to get your tow under it with the bike not running and in first gear lift the shift lever you may have to rock the bike a little to get the gears to mesh. what you're looking for is the shift lever hitting/ touching / rubbing on some other part of the bike (doesn't take much) tiny bend in the shift lever can cause this type of problem but if all seems clear.........
I so know that you probably don,t want to hear this so don,t hate the messenger but probably the shifter forks are worn / bent and need replacing ...before you ask no not easy to do good luck
ride safe