Your sentence is: The jungle is a hot, rainy, thick forest.
This sentence is mostly correct but we can make it sound even better by adding the word 'a' before 'hot' because we are describing the jungle as 'a hot, rainy, thick forest'. The corrected sentence is:
The jungle is a hot, rainy, thick forest.
Actually, your original sentence already includes 'a' correctly at the start: 'The jungle is a hot, rainy, thick forest.'
So, your sentence's grammar is correct!
Great job describing the jungle with different words that tell us how it is (hot, rainy, and thick).