If it hadn't been 10.11 Public Beta (2) on the main volume, I would have advised to boot to Internet Recovery Mode (IRM) (altcmdR) or a bootable thumb drive (BTD), just delete the 3rd partition with
gpt remove -i 3 /dev/disk0 and fully expand the main partition with Disk Utility. After rebooting to the main volume I would have reinstalled the latest OS X (Yosemite 10.10.4 as of this writing) which recreates the Recovery HD while preserving all non-Apple apps and user-created data.
Instead you have to
dd the Recovery HD from a second Mac running Yosemite. Copy the resulting file to the first Mac. Then boot Mac1 to IRM/BTD, delete the 3rd partition with gpt, reboot to IRM/BTD, resize the main volume with
diskutil resizeVolume /dev/disk0s2 size, create a 3rd partition with:
gpt add -b first_freeblock -i 3 -s 1269536 -t 426F6F74-0000-11AA-AA11-00306543ECAC /dev/disk0
dd the previously made copy of Recovery HD (Mac2) back to /dev/dis0s3.
diskutil ... size you have to do some math considering that the last blocks of a Yosemite drive containing the Recovery HD usually look like this:
.... 2 GPT part - 53746F72-6167-11AA-AA11-00306543ECAC
975503592 1269536 3 GPT part - 426F6F74-0000-11AA-AA11-00306543ECAC
976773135 32 Sec GPT table
976773167 1 Sec GPT header
Recheck this with
sudo gpt -r show /dev/disk0 on Mac2.
The example here is a 500.1 GiB (976773168 blocks à 512 B) disk.
Accordingly "size" has to be smaller than or the same size as (total blocks - 409640 - 1269576 blocks). The second term is block0 - block409639 containing PMBR, the primary GPT header and table and the EFI partition. The third term is the size of the Recovery HD and the secondary GPT header and table. Please consider that block counting starts with 0: block 0 is the the first block or block 975503592 is the 975503593rd block.
If you are to lazy to do the math, you may first create the third partition and
dd the Recovery HD file and then - after a restart - expand the main volume using Disk Utility.
A second more risky method (not risky regarding your data on the main volume but I'm not sure if it is successful) is just moving the partition borders of disk0s3. This method presumes that no files of the old Recovery HD were moved "physically" on disk0s3 after deleting the Boot Camp partition with the Boot Camp assistant.
Boot to IRM/BTD and enter in Terminal:
gpt remove -i 3 /dev/disk0
and reboot to IRM/BTD. Then add a third partition with the following command (already adapted to the size of your disk!):
gpt add -b 975835488 -i 3 -s 1269536 -t 426F6F74-0000-11AA-AA11-00306543ECAC /dev/disk0
and reboot to IRM/BTD. Open Disk Utility and verify/repair the hard drive (and not the main volume!). Then enlarge the main volume to max size and verify /repair the main volume. Finally try to boot to Recovery Mode (cmdR).