Corrected sentence:
Elephants use their trunks to smell, to hold things, and even as snorkels when swimming.
Why this is better (step by step):
- Their trunks — The original said "trunk." Because "elephants" is plural, use the plural possessive "their trunks" (each elephant has its own trunk).
- Parallel structure — Your list mixed forms ("to smell," "to hold things," and "as a snorkel"). Make the parts match. In the corrected sentence we keep the pattern "to ..." for the first two items and use "as snorkels" for the third so the meaning stays clear and the grammar matches.
- Snorkels (plural) — Since elephants is plural, use "snorkels" (plural) rather than "a snorkel."
- Comma and "even" — The comma before "and" is fine for a list of three. Placing "even" before "as snorkels" emphasizes that this use is surprising.
Other good versions:
- Elephants use their trunks to smell, hold things, and even to breathe like a snorkel when swimming.
- Elephants use their trunks for smelling, holding things, and as snorkels when swimming.
Any of these are grammatically correct; pick the one that sounds best to you.