Google 3" to 2.5" adapter SSD laptop
What make and model laptop?
There are adapters and options but you will need to take a close look at how the existing HDD is installed inside the laptop.
Then compare the screw holes, dimensions, and overall physical fit of any adapters you find.
Two things need to work/happen:
1) the SSD must be able to securely fit into the adapter without any blocking of the data and power cable connections.
2) the adapter (with SSD installed) must securely fit into the laptop "dock" that originally hosted the HDD.
Take a close look at the existing HDD installation. How is the drive currently installed - how many screws, how many holes are available for screws, how far apart are the holes.
And keep in mind any air flows that are in place - you do not want to block any cooling....
Likely converting to a SSD is doable but the details matter.