Learn English – Let’s form groups of 3 or 4 people

Neither option sounds quite right unless you have groups named "A", "B" and "C".

When "A", "B" and "C" are being used as nouns (as is common in the Football World Cup), then favour "group" at the start.

England, Brazil, Spain and France from Group A will continue to the next round.

I've had to think about the word group, but I think the pattern is: If you are using group with an adjective, put the adjective in front. If you are using it with a noun, put the word group in front and add "of"

I joined my local gymnastics group.

I have joined a group of gymnasts.

For your example specifically, you should consider something like:

You should consume [more] from the tropical fruits food group.

Here, the world food group is a compound noun. I'm not sure it works perfectly as "tropical fruits" isn't one of the standard food groups.

Suppose we've got a function called showMessage then which one do you use? "You should use showMessage function." or "You should use function showMessage."

Here I would use the first option, but add a definite article.

You should use the showMessage function.

I saw in many cases that people say: "Variable x is used for ..." so, why wouldn't we say "x variable is used for ..."?

I can't think of the rule here, but Variable x is the correct way to say this. You can omit the word variable completely:

The tutorialMetadata variable is used to store information read from an XML file.

tutorialMetadata is used to store information read from an XML file.