When I do:
sfdisk -uM /dev/mmcblk1 << EOF
,3500,C
,,L
,,
,,
EOF

and then:

sfdisk -l

I see the table, it list:
mmcblk1p1 as a W95 FAT32 (LBA) 3583999+
mmcblk1p2 as LINUX 4177920
mmcblk1p3 Empty
mmcblk1p4 Empty

So I assume that works and simply sets partition 1 as close as possible to 3.5gb. Leaving the rest as partition 2.
How do I partition (split) p2 to give me p3 remaining?
or can I just set p3? I don't know how..