Vežbam tri dana tokom nedelje.

Because after the numbers 2, 3, and 4, Serbian usually uses a special form of the noun: dva/tri/četiri + noun in the paucal form, which often looks like the genitive singular.

So:

• jedan dan = one day
• dva dana = two days
• tri dana = three days
• četiri dana = four days

But:

• pet dana = five days
• šest dana = six days

So tri dana is the correct form.

Because tokom requires the genitive case.

The base form is nedelja.
After tokom, it becomes nedelje:

• tokom nedelje = during the week

This is a normal case pattern in Serbian:

• tokom dana = during the day
• tokom leta = during the summer
• tokom nedelje = during the week

It can mean either week or Sunday depending on context, which is something English speakers often notice.

Here, because of tokom and the overall meaning, nedelje means week:

• tokom nedelje = during the week

If someone meant during Sunday, that would normally be expressed differently and the context would make it clearer.

Serbian often leaves out subject pronouns when they are already clear from the verb ending.

Vežbam means I practice / I exercise / I train.
The ending -am shows 1st person singular, so ja is unnecessary.

You could say:

• Ja vežbam tri dana tokom nedelje.

But that usually adds emphasis, like I exercise three days during the week.

Vežbam comes from vežbati and can mean several related things, depending on context:

• I exercise
• I practice
• I train

If the sentence is about fitness, it most naturally means I exercise or I work out.
If it is about a skill, instrument, language, etc., it could mean I practice.

So the verb is broader than one single English verb.

In Serbian, the present tense is commonly used for habitual actions—things you do regularly.

So Vežbam tri dana tokom nedelje means something like:

• I exercise three days during the week.
• I exercise three days a week.

It describes a routine, not necessarily something happening right now at this exact moment.

Because the sentence describes a repeated, ongoing habit.

Vežbati is an imperfective verb, which is the normal choice for:

• habits
• repeated actions
• general activities
• ongoing processes

Since the speaker is talking about a routine, imperfective is exactly what you would expect.

Yes, it is understandable and grammatically fine, but many native speakers might more naturally say:

• Vežbam tri puta nedeljno. = I exercise three times a week.
• Vežbam tri dana nedeljno. = I exercise three days a week.

The original sentence Vežbam tri dana tokom nedelje is still possible, but it sounds a bit more literal, like emphasizing that those exercise days happen within the week.

This is a useful distinction:

• tri dana tokom nedelje = on three days during the week
• tri puta nedeljno = three times a week

These are often similar, but not always identical.

For example:

• If you work out once on Monday, once on Wednesday, and once on Friday, both expressions fit.
• If you work out twice on one day and once on another day, then tri puta nedeljno is true, but tri dana tokom nedelje is not.

So dana focuses on days, while puta focuses on number of times.

Not here.

In this sentence, tri dana means on three days, not for 72 continuous hours.

So the meaning is about frequency across the week, not duration of one uninterrupted activity.

Context matters a lot in Serbian, just like in English.

Yes. Serbian word order is flexible, and changing it usually changes emphasis, not the basic meaning.

For example:

• Vežbam tri dana tokom nedelje.
• Tokom nedelje vežbam tri dana.
• Tri dana tokom nedelje vežbam.

All of these can work, but the first one is a neutral choice.
Putting tokom nedelje first emphasizes during the week.

A rough pronunciation is:

• VEZH-bam

A few notes:

• ž sounds like the s in measure or the g in genre
• the stress is usually on the first syllable: VEŽbam

So it is not pronounced like an English z.

Yes. Tokom nedelje and u toku nedelje are very close in meaning.

Both mean during the week.

• tokom nedelje
• u toku nedelje

Tokom is a bit more compact.
U toku is also very common and natural.

Not as naturally for this meaning.

If you want to say during the week or in the course of the week, tokom nedelje or u toku nedelje is much better.

For expressing frequency, native speakers often prefer:

• nedeljno = weekly
• tri puta nedeljno = three times a week
• tri dana nedeljno = three days a week

So u nedelji is not the most natural choice here.