Are these sentences correct?

[shb27]

Peninsula is a spacious piece of land, which seas surround from three sides.

Peninsula is a spacious piece of land, seas surround it from three sides.

Peninsula is a spacious piece of land, which is surrounded by seas from three sides.

Are the previous sentences correct?

 

[Raymott]

A peninsula is a spacious piece of land which seas surround [STRIKE]from[/STRIKE] on three sides.

A peninsula is a spacious piece of land. Seas surround it [STRIKE]from[/STRIKE] on three sides.

A peninsula is a spacious piece of land which is surrounded by seas [STRIKE]from[/STRIKE] on three sides.

Are the previous sentences correct?

None are correct. Delete all the commas; you're using defining clauses. You need an article before 'peninsula'.
In 2, you can't join two sentences with a comma.

 

[Rover_KE]

Also, delete 'spacious'. It is meaningless.

A peninsula can be as big as Florida or as small as a few square metres.

Additionally, change 'seas' to 'water' as lakes and oceans have peninsulas, too.

* * *

A better title would have been 'peninsula definition'.

Extract from the Posting Guidelines:

'Thread titles should include all or part of the word/phrase being discussed.'

Rover