A window with no glass

A lens is like a window but the lens of an eye has no glass.

A student with no class

Another word for student is pupil but the pupil of an eye has no class.

A sea in any season

Iris means rainbow which is shaped like a C (“sea”). The iris of the eye blooms all year round.

An epistle? No, a threesome

Another word for epistle is letter. An eye is three letters.

The title: An easy positive riddle

Eye sounds like “aye”, a positive answer, the sí of the sea.

A window with no glass

A lens is like a window but the lens of an eye has no glass.

A student with no class

Another word for student is pupil but the pupil of an eye has no class.

A sea in any season

"A sea" sounds like "I see", which the eye does year round.

An epistle? No, a threesome

Another word for epistle is letter. An eye is three letters.

The title: An easy positive riddle

"Easy" sounds like "I see". Eye sounds like “aye”, a positive answer, the sí of the sea.