Is there a way to delete the last 3 backups from Time Machine?
To answer your direct question, yes, you can delete backups by going into Time Machine, clicking on the backup in question (on the right-hand column), then right-clicking on your computer's name in the sidebar in the Finder window and choosing Delete Backup.

You should be able to downgrade without deleting backups, though. In order to do so, you'll need to do a few things.

First, disconnect your Time Machine drive so nothing happens to it during the following process.

Next, if you have any critical files that were created since you installed Sierra, copy them to another drive or a cloud service like Dropbox.

At this point you'll need to create a bootable USB flash drive with the El Capitan installer on it.

Download the El Capitan installer from the Mac App Store again. When's it done downloading it will try to run, and that will cause an error that it can't be run - don't worry about it. Just move to the next step.

Format the USB flash drive as HFS+ and name it whatever you like, but don't include spaces - the example used here is ElCapInstaller. If you change the name of your drive, change that name where it appears in the below command.

Once you have your USB flash drive ready, create the installer by opening Terminal and copying and pasting the following command:

sudo /Applications/Install\ OS\ X\ El\ Capitan.app/Contents/Resources/createinstallmedia --volume /Volumes/ElCapInstaller --applicationpath /Applications/Install\ OS\ X\ El\ Capitan.app --nointeraction


(Again, replace ElCapInstaller with the name of your USB flash drive if you changed it.) This may take up to 30 minutes or more but it will tell you in Terminal once it's complete by showing "Copy Complete" and then "Done". Wait for "Done".

Reboot your computer with the flash drive still attached and hold Option as it's starting up. You will be shown options for startup disks. Select the flash drive you previously created by double-clicking on it.

At this point you want to reinstall El Capitan by erasing your hard drive. Otherwise you won't be able to proceed. Remember, you have a Time Machine backup in case anything goes wrong.

An OS X Utilities window will appear. Select Disk Utility and click Continue.
Once it launches, select the Mac’s hard drive in the left column. Click the Erase tab, and in the windows that appears, name your hard drive and select one of the OS X Extended formats, then click the Erase button to wipe out everything on the hard drive.

Once Disk Utility is done reformatting, quit the app. This will take you back to the OS X Utilities window.

Select Install OS X, click Continue, and proceed through the installation process.

Once the installation is complete and you're at the desktop, re-connect your Time Machine drive and run Utilities/Migration Assistant in your Applications. This will walk you through the procedure of restoring your files from your Time Machine backup. Obviously you will need to select one from before you upgraded to Sierra.