Partition space with the command diskutil partitionDisk
Note: I made the assumption you would want the volumes to be journaled. So I used jhfs+ instead of hfs+.


The command you are looking for is given below.

diskutil partitiondisk disk2 jhfs+ 14 15887144s jhfs+ 15 15887144s jhfs+ 13 R


To have 3 partitions of 8G and one with the remaining space, enter the following command.

diskutil partitiondisk disk2 jhfs+ 14 15887144s jhfs+ 15 15887144s jhfs+ 13 15887144s jhfs+ Combo R


Actually, the command you entered reserved exactly 8 GB of space for the 14 and 15 volumes. However, this includes 134,217,728 bytes of free space between each hfs+ formatted partition. These kinds of obscure requirements generally go away when APFS is used. 

Here is how I determined the correct value was 15887144s. The sector size is 512 bytes. So 134,217,728 bytes is 262,144 sectors and 8,000,000,000 bytes is 15,625,000 sectors. Add these two values together and you will get 15,887,144 sectors. 

The amount of free space, between each partition, can be determined from the output of the command given below. This is how I came up with the value of 262,144 sectors.

gpt -r show /dev/disk2


The sector size can be found by entering the command given below.

diskutil info disk2 | grep "Block Size"


Also, if the output from the gpt -r show /dev/disk2 command shows a Pri GPT table size of 32 sectors, then the sector size is 512 bytes. Otherwise, you can assume a sector size of 4096 bytes.