Question: What I know about metabolism is that,during exercise body uses carbohydrate first then fats and then protein,so my question is why does body prefer glucose first as fuel for energy and what makes glucose biochemically preferred molecule to be metabolized first

Answer: 
None of them can be directly use for energy. We all talk about aerobic oxidation, which is the major process for metabolism.

They all need to go into the tricarboxylic acid cycle (TCA cycle) and oxidation phosphorylation to produce ATP, which is the energy used in organism.

For glucose, they can break down into two pyruvate and enter the TCA cycle. That is the fastest pathway organism use to produce energy.

For fat, since they have long carbon skeleton chain, they cut off 2 carbon compounds from the fatty acid every time. And under a series of process, it become pyruvate and go into the TCA cycle. So it really takes long time and complicated pathway to oxidate the whole fatty acid molecules.

For protein, it is much more complicated. Since protein is not energy store form, it need to become a energy store form first. First it need to be degraded into amino acid, and different amino acids takes different procedure to go into the TCA cycle. Here is the picture I found on the book by John L. Tymoczko, Jeremy M. Berg and Lubert Stryer.

[![Fates of the carbon skeleton of amino acids](https://i.stack.imgur.com/ySsh1.png)](https://i.stack.imgur.com/ySsh1.png)

In sum, glucose is the fastest energy form that body can use to take metabolism, oxidate fatty acid or proteins take much more complicated pathway.
I already forget all of the structure of carbohydrate, fat and also protein. But I still remember the reason for body using order of these compounds is related to the structure. 

None all of carbohydrate can be directly used, only simple carbohydrate, like glucose, can be directly used.

For fat, it can be turn by carbohydrate for storage, and of course they are storage form, then they won't be easy for using. Even fat need to be used, they will turn to be carbohydrate again then using.

For proteins are another story, they aren't storage form, but they have some ability to form carbohydrate. So if the body is very starving and there is no other resource, the body will begin turn the protein to carbohydrate for using.