Den tredje bussen kommer.

Because Norwegian usually uses double definiteness when a noun is definite and has an adjective or ordinal in front of it.

So:

• buss = bus
• bussen = the bus
• den tredje bussen = the third bus

You need:

• den before the adjective/ordinal
• the definite ending -en on the noun

This is normal in Norwegian.
If there were no adjective or ordinal, you would usually just say bussen.

Because buss is a masculine noun in Bokmål.

The definite markers are:

• den for masculine and feminine singular nouns
• det for neuter singular nouns
• de for plural

So:

• den tredje bussen = the third bus
• det tredje huset = the third house

Since buss takes -en in the definite singular (bussen), it goes with den.

Tredje is an ordinal number. Ordinals are words like:

• første = first
• andre = second
• tredje = third
• fjerde = fourth

They are used when you are talking about position or order, not quantity.

So tre means three, but tredje means third.

Usually, tredje stays tredje in common use.

For a learner, the important thing here is that:

• den tredje bussen
• det tredje huset
• de tredje bussene

all use tredje.

So unlike many ordinary adjectives, this ordinal does not create much extra visible change here. The main thing you need to notice in this sentence is the definite pattern around it.

Kommer is the present tense of å komme.

In English, it can match different ideas depending on context:

• comes
• is coming
• arrives

So this sentence could mean something like:

• the third bus comes
• the third bus is coming
• the third bus arrives

Norwegian present tense often covers both simple present and present progressive, where English makes a bigger distinction.

Because Den tredje bussen is the subject, and kommer is the finite verb.

In a basic Norwegian main clause, the finite verb is normally in second position. This is the well-known V2 pattern.

Here, the first element is the whole subject phrase:

Den tredje bussen + kommer

So the verb comes right after that phrase.

You can see the same rule if another element comes first:

• Nå kommer den tredje bussen.

Here nå is first, so kommer still stays in second position.

Most naturally, den tredje bussen means the third bus in a sequence or order.

For example:

• the third bus to arrive
• the third bus in a line
• the third bus on a list

If you mean route number 3, Norwegian would more naturally say something like:

• buss nummer tre
• 3-bussen

So this sentence is usually about position, not bus number.

In standard Bokmål, that would usually not be correct for this meaning.

If the phrase is definite and has an adjective or ordinal before the noun, you normally need the extra determiner:

• den tredje bussen ✅

Without den, it sounds incomplete or nonstandard in ordinary Bokmål.

A simple learner-friendly approximation is TRED-ye.

A few helpful points:

• the first part sounds like tred
• the dj combination is not pronounced like a hard English j in all dialects
• many learners hear it as something close to TRED-ye

You do not need perfect dialect pronunciation right away. If you say something close to TRED-ye, people will usually understand you.

Yes, it is completely natural.

It sounds like something you would say when identifying one bus among several, especially by order:

• maybe several buses are expected
• maybe you are counting which one arrives next
• maybe you are comparing a first, second, and third bus

So grammatically and stylistically, the sentence is very normal.