Eu vou pagar a mensalidade em três vezes.

Vou pagar is the very common near future construction in Brazilian Portuguese: ir + infinitive.

• eu vou pagar = I’m going to pay
• eu pagarei = I will pay

Both are correct, but vou pagar sounds more natural and conversational in everyday Brazilian Portuguese.
Pagarei is more formal, more written, or sometimes more emphatic.

Mensalidade usually means a monthly fee, monthly payment, or tuition installment, depending on context.

Common examples:

• school tuition paid monthly
• gym membership fee
• club fee
• course fee

So it is not just any random payment. It usually refers to something that is charged every month.

The a is the definite article, meaning the.

• a mensalidade = the monthly fee
• mensalidade = monthly fee in a more general sense

Portuguese uses definite articles more often than English does, so a mensalidade sounds very natural.

Here, em três vezes means in three installments.

Literally, vezes often means times, but in Brazilian Portuguese, especially in shopping and payment contexts, pagar em X vezes is a very common way to say that something will be split into that many payments.

So:

• em três vezes = in three installments
• not literally three times in the English sense

This is extremely common in Brazil, especially with credit cards, tuition, and other recurring charges.

Yes, vezes usually means times, as in:

• três vezes por semana = three times a week

But in Brazilian payment language, vezes has a special everyday use:

• pagar em duas vezes
• parcelar em cinco vezes
• em dez vezes sem juros

In these situations, vezes means something like payments/installments. It is idiomatic and very common.

Yes. Em três parcelas is also correct and often a little more explicit.

Compare:

• em três vezes = very common, everyday Brazilian usage
• em três parcelas = also common, slightly clearer because parcelas directly means installments

Both are natural. In many real-life situations, Brazilians use either one.

Because pagar em três vezes is the normal pattern for saying a payment is divided into installments.

• pagar em três vezes = pay in three installments
• pagar três vezes would usually mean pay three times

So em is important because it signals the idea of division into installments, not repeated action.

Yes, absolutely. In Brazilian Portuguese, subject pronouns are often omitted when the verb already makes the subject clear.

So both are natural:

• Eu vou pagar a mensalidade em três vezes.
• Vou pagar a mensalidade em três vezes.

Including Eu can add emphasis, contrast, or clarity, but it is not required here.

Yes, it is natural. A Brazilian speaker would understand it immediately.

That said, in everyday speech you may also hear:

• Vou pagar a mensalidade em três parcelas.
• Vou parcelar a mensalidade em três vezes.
• A mensalidade vai ficar em três vezes.

Your sentence is perfectly normal, especially in a payment context.

They are close, but not exactly the same.

• vou pagar a mensalidade em três vezes = I’m going to pay the monthly fee in three installments
• vou parcelar a mensalidade em três vezes = I’m going to split the monthly fee into three installments

Parcelar focuses more on the act of dividing the amount.
Pagar em três vezes focuses more on how the payment will happen.

In practice, both are very common and often used in similar situations.

Because after em, the phrase em três vezes works as a fixed expression meaning in three installments.

You normally would not say em as três vezes here.
Just use:

• em duas vezes
• em três vezes
• em seis vezes

This is the standard pattern.

In Brazilian Portuguese, it is roughly:

men-sah-lee-DA-jee

A few pronunciation notes:

• the stress is on da
• the final de often sounds like jee in many Brazilian accents
• the s in mensalidade sounds like s, not z, in most standard pronunciations of this word

A rough English approximation is helpful, but listening to native audio is best.

Not normally. Because of em três vezes, the natural meaning is in three installments.

If you wanted to say I will pay the fee three times, you would normally say something more like:

• Vou pagar a mensalidade três vezes
• or use a fuller context to make the repetition clear

So the preposition em strongly points to the installment meaning.

No. It often refers to tuition, but not only tuition.

It can also be:

• a gym fee
• a subscription fee
• a club fee
• a monthly service charge

The exact meaning depends on context. The core idea is simply a recurring monthly charge.

A very natural translation would often be:

• I’m going to pay the monthly fee in three installments.

Depending on context, you could also say:

• I’m going to split the monthly fee into three payments.
• I’ll pay the tuition in three installments.

Even though the Portuguese uses vezes, the best English translation in most cases is installments or payments, not times.