Það eru þrír dagar síðan við sáum svona rólegar bylgjur.

Here það is a dummy subject, much like English it in It has been three days. It does not point to any specific thing.

So Það eru þrír dagar... is a very natural Icelandic way to introduce a time statement like It has been three days...

A very literal breakdown is:

• Það eru = there are / it is
• þrír dagar = three days
• síðan... = since...

So the whole structure is basically It is three days since..., which English usually expresses as It has been three days since...

Because þrír dagar is plural.

The verb vera often agrees with the noun phrase that follows in this kind of sentence. Since dagar is plural, Icelandic uses eru:

• Það er einn dagur síðan... = It has been one day since...
• Það eru þrír dagar síðan... = It has been three days since...

So eru matches the plural idea of three days.

Because in this sentence þrír dagar is not being used as a simple duration expression like for three days. It is part of the structure It has been three days since..., where the time phrase appears in the nominative.

That is why you get:

• þrír dagar = nominative plural

Compare that with a true duration phrase:

• Við biðum í þrjá daga = We waited for three days

There, þrjá daga is accusative because it is part of a different kind of time expression.

So:

• Það eru þrír dagar síðan... = Three days have passed since...
• í þrjá daga = for three days

These are different constructions.

In this sentence, síðan means since.

It introduces the event that happened in the past:

• síðan við sáum svona rólegar bylgjur = since we saw such calm waves

A useful thing to know is that síðan can have a few related meanings in Icelandic, depending on context:

• since
• afterwards / then
• sometimes part of expressions involving ago

But here the meaning is clearly since.

Because síðan við sáum... is a subordinate clause.

In main clauses, Icelandic often follows the V2 pattern, where the finite verb comes early in the sentence. But after a subordinating word like síðan, the clause usually has more ordinary subject-verb order:

• við sáum = we saw

So this is exactly what you would expect after síðan.

Sáum is the 1st person plural past tense of sjá, meaning to see.

So:

• ég sá = I saw
• þú sást = you saw
• við sáum = we saw

This verb is irregular, so learners usually just have to get used to its forms:

• sjá = to see
• sá = saw
• séð = seen

In this sentence, við sáum simply means we saw.

Because this is the direct object of sáum, and it is feminine plural accusative.

Here is the logic:

• bylgja = wave
• it is a feminine noun
• plural bylgjur = waves
• after sáum, the object is in the accusative
• the adjective must agree with the noun in gender, number, and case

So:

• bylgjur = feminine plural accusative
• rólegar matches that same gender, number, and case

That is why you get:

• svona rólegar bylgjur = such calm waves

Svona here means something like like this, this kind of, or such.

In this sentence, the most natural English sense is:

• svona rólegar bylgjur = such calm waves
• more literally: waves this calm / waves like this

It adds the idea that the waves were calm to this particular degree or in this particular way.

So rólegar bylgjur is just calm waves, while svona rólegar bylgjur is more like such calm waves or waves this calm.

Yes, very normal.

Það eru þrír dagar síðan... is a standard and idiomatic way to express that a certain amount of time has passed since something happened.

English often uses the present perfect in the first part:

• It has been three days since...

Icelandic commonly uses the present tense of vera in this pattern:

• Það eru þrír dagar síðan...

So even if the wording looks a bit different from English, the structure is completely natural in Icelandic.