That's true bitcoin is more popular and widely used, the market cap of bitcoin is huge as compared to dogecoin and litecoin, and people have more faith in bitcoins as compared to others.

Think of it this way.

Bitcoin is the cock, litecoin sucks the cock and dogecoin licks the balls. They are all working together to spray cryptocurrency all over the world.

i always admire LTC and Doge since they could maintain their market cap and volume for this long without having to give any new and useful feature.

Bitcoin is far better for me as compared to the others, as bitcoin has given me some good earning opportunities, so if I have to invest my money then I would prefer to invest into bitcoins.

They are all different (mostly).
They have different developers however both use similar programs/code taken from Bitcoin.

Litecoin and Dogecoin use a SCRYPT algoritm for mining.
Bitcoin uses an SHA-256d algorithm or mining.

This means that the coins and software are slightly different to encorporate this.
I think that DoOGECOIN is the most likely to profit now, followed by Bitcoin and then Litecoin.

Exactly bitcoin is the king when it comes to digital currency as people are more attracted towards bitcoin and have more faith in it as compared to any other digital currency.

They are a bit different technologically.

Bitcoin is the leader. Litecoin, dogecoin are the photostats of bitcoin with varying technology.
Dogecoin & litecoin sre the alternate coins. Among this bitcoin is the most secure.