Is it safe for my 3 month old baby to sleep swaddled and flat in a pram bassinet for extended periods (for example, overnight)?

Product Safety Australia & Red Nose do not recommend a pram as a sleep environment, particularly overnight when not supervised, as they do not have standards as sleep environment.

Avoid sleeping children in prams or strollers…....If they are asleep, maintain regular supervision.
https://www.productsafety.gov.au/standards/prams-strollers

Never leave your baby unattended in a pram or stroller. It is not a substitute for a cot
.https://rednose.org.au/article/prams-and-strollers

As far as swaddling is concerned, Red Nose recommends the following:
If you wrap your baby, consider baby’s stage of development. Leave arms free once the startle reflex disappears around 3 months. Most babies eventually resist being wrapped. Wrapping style should be appropriate for the
baby’s developmental stage.
It is essential to discontinue wrapping as soon as baby starts showing signs that they can begin to roll, usually between 4-6 months of age but sometimes younger.
https://rednose.org.au/article/is-it-safe-to-wrap-swaddle-my-baby

Red Nose recommends the safest place for baby to sleep is in a cot that meets Mandatory Australian Standards.