3Pk???

Actually Macbeth, that is a little trick I have been known to use with Warbands.

Another trick is to use the rear fast moving elements as a separate battleline.
For 1 PIP the front line all moves into contact, then, depending on PIPs available,
fast elements from the second line move up individually or in small groups to
back up the front line and form rear supporting columns...all in a single bound.
This allows you to give extra punch where it will be most effective, prevents your
opponent from guessing what you are up to, and keeps some elements out of
X-Ray Threat Zones to form a nice handy reserve (I like having reserves).
Light Horse could also do the same.

Anyway, getting back to the issue at hand.
I think the thing that is muddying the waters is their name.
Calling them ‘fast 3Pk’ brings along with it all the mental baggage associated with Pikes.
But they are not Pikes…they are just spears being held with both hands.
If merely holding a spear with both hands allows a +3 for rear support, then I want my
Hoplites to drop their shields and grab their pointy sticks with both mitts!

Calling them what they really are, ‘fast 3Sp’, gives us a clearer insight into their true nature.
And I have no problem giving them +1 for rear support in good going against foot (just like Wb).

Of course, DBA is an abstract wargame, and we have to tale play balance into consideration.
So perhaps it would have been better to have these enigmatic troops as ‘fast 3Sp’:-
     Let them have a CF of 3 against foot and a CF of 4 against mounted (unlike Ax)…
     Being Sp they would not pursue and get themselves into trouble (unlike Pk)…
     Being Sp they would still be vulnerable to Wb quick kills (unlike Ax)…
     Being 'fast' they would not be slowed or suffer the -2 penalty for being in bad going…
     Being 'fast' they’d still recoil on an equal score from solid foot…
     And let them have +1 for rear support in good going against foot (but not against Ps).

So there you have it…’fast 3Sp’ as a sort of beefed-up Ax, but still vulnerable to Wb,
that are as good as Ax in rough or bad going, better than Ax against mounted, and
better than Ax in good going with rear support against foot (but with a shorter battleline).

"Fast elements neither give nor receive flank support" - page 11, second paragraph.
(But fast Warbands can give and receive rear support from other Warbands.
Let fast 3Sp do the same with other fast 3Sp)

…and another thing…

Phil Barker in his very own “Armies and Enemies of Imperial Rome” shows the Picts
carrying small square shields and ‘long’ spears (but no longer than Greek ones).
And the only reference to holding the spear in both hands appears to come from
a single stone showing one warrior holding off some rather Roman looking cavalry.

It seems to me to be a huge leap of the imagination to assume that ALL Picts did this,
and that holding a mere 9 foot/3 metre spear with both hands makes you almost as
good as Alexander’s Macedonian sarissai armed phalangites or the later Swiss with
their 15-18 foot/5-6 metre long pikes!
                      🤪

It's hard when we don't know the prototype for the troop element. Was it the Welsh, the Hittites or the Japanese? If they're Spear then they are base 4 CF against foot and mounted. Maybe that's enough? No side-support (as per page 11). No rear support. Superior mobility. Fight at CV 2 in bad going. It just may do the trick.

Jim

Don’t forget they are supposed to be vulnerable to shooting as well.

True solid 4Sp and all Bd are CF 4 when shot at, partly for their large
shields, and partly for their body armour (even having a leather or
stiffened linen spolas helps to protect against missiles).

True solid 4Pk and all Ax are CF 3 when shot at or when fighting Ps.
These ‘fast 3Sp’ should also be CF 3 against shooting and skirmishers.

Making them CF 3 against foot but with +1 for rear support (like Wb)
does just that.

But all this is for House Rules or future versions of DBA.
At the moment we’ll just have to live with the fantasy of holding a mere
spear with two hands somehow makes you as good as a true pikemen.

Most of the case for the rear support comes from Phil himself and the classification of such troops in DBM as Ax (X), by the original rules in DBM Ax (X) were to count as Ax(O) vs mounted troops and could count another element as rear support. If this has changed in DBMM to Ax(X) counting as Ax(S) against mounted I don't know.
However, there are grounds (though speculative) that such troops did develop deeper formations in response to new threats....Akkadians/Summerians vs Chariots and the North Welsh for example vs the Normans....the Picts/Scots too were most likely using deeper formations prior to William Wallace. Yes this is speculative but probable and its not the only such example in the DBA universe.
Maybe keeping the 3Sp from 2.2 without rear support may have been a better option?.. But this does not allow for the vulnerability to shooting, so the classification as 3Pk makes the element more vulnerability than Sp to shooting, but maybe the rear support may make them too powerful..a mere +1 (in good going) for rear support vs Bd,HD,Pk,Sp,Wb or Mounted could be more realistic?

I think Jim has it pretty right.

In LADG there is no 'rear support'. Sure, a unit of Pike is usually modeled deeper with an extra rank or even two, but it counts as one unit just like everything else.
If Pk had their CF adjusted so that they did not need rear support, it would de-nerf Pk armies. Just need to get the CF balanced right.