Kernel 4.0 introduced a different processing for Synaptics PS/2 devices:
https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/drivers/input/mouse/synaptics.c?id=e9e8520f229bd6881b51d03c010df6c0312bfef8

This commit introduced a different behavior when dropping a third finger on the sensor (see also https://lkml.org/lkml/2015/6/11/487):

When we have 2 fingers and when we drop a third, the sequence is the following:

(slot 0 contains a touch in 200,2000 and slot 1 an other in 3000,3000):
ABS_MT_SLOT          0
ABS_MT_POSITION_X    4000
ABS_MT_POSITION_Y    4000
ABS_MT_PRESSURE      78
ABS_MT_SLOT          1
ABS_MT_TRACKING_ID   -1

ABS_X                4000
ABS_Y                4000
ABS_PRESSURE         78
BTN_TOOL_DOUBLETAP   0
BTN_TOOL_TRIPLETAP   1
--- SYN_REPORT (0) ----------
ABS_MT_SLOT          0
ABS_MT_POSITION_X    4000
ABS_MT_POSITION_Y    4000
ABS_MT_PRESSURE      78
ABS_MT_SLOT          1
ABS_MT_TRACKING_ID   55
ABS_MT_POSITION_X    2000
ABS_MT_POSITION_Y    2000
ABS_MT_PRESSURE      72

ABS_X                4000
ABS_Y                4000
ABS_PRESSURE         78
--- SYN_REPORT (0) ----------

----- 3 fingers release -----

--- SYN_REPORT (0) ----------
ABS_MT_SLOT          0
ABS_MT_TRACKING_ID   -1
ABS_MT_SLOT          1
ABS_MT_TRACKING_ID   -1

BTN_TOUCH            0
ABS_PRESSURE         0
BTN_TOOL_TRIPLETAP   0
--- SYN_REPORT (0) ----------

We thus get a slot release when BTN_TOOL_TRIPLETAP is set to one.

For Synaptics devices reporting in PS/2 mode, we should have a special case where we ignore the release of the slot when TRIPLETAP is set and DOUBLETAP is unset.