On the images I counted 63 vehicles that could match size with PHL-03 arranged in 3 groups of 21 vehicles
You have brought all issues logically.
You have correctly identified 63 PHLO3 sized vehicles which are there together.
Firstly, it is generally known and accepted, that PLA's Long Range MLRS Battalions have 12 launchers each - as '78' has also agreed. So, 12 launchers requires 12 replenishment vehicles minimum. So, every battalion should ideally have minimum 24 PHL03 sized vehicles. Which means two PHL03 Battalions, one PCL191 battalion, that is three MLRS Battalions. While photos & images bring out two PCL181 Battalions, there is minimum one PL66 battalion (guns are lying away from the gun towing vehicle). So, there are six battalions in the brigade of Tibet Military Region Artillery Brigade.
This is purely from a representation of vehicles displayed outside the sheds. Now, if you see the numbering series displayed, with the case of 77th Group Army Artillery brigade (LX41), the vehicle numbers clearly identified for the PHL03 battalion were 5206, 5207 & so on indicate that PHL03 is the fifth battalion.
The US ATP talks about four battalions but doesn't include PCL191 battalion.
Induction of both PHL03 & PCL181 happened earlier as part of Mechanisation of Group Army's Artillery Brigades. Two battalions of each generally got inducted. PCL191 is the latest addition which makes it fifth battalion.
While many Chinese photos continue to show the PL66 or PLZ05 and PLZ05A (inducted to replace Type59 130mm), very few showed 130 mm some time back. So, as this equipment is being phased out, PLZ05C & Type 66 battalions are there which make the 6th battalion.
So, with every induction of PCL191 or PCL181 (as still happening in Division Artillery Brigades of Xinjiang Military Region), the composition of Artillery Brigade is improving as planned as part of Mechanisation and Informatisation.
The combinations thus exists in my appreciation is
A) 3-4 Howitzer battalions- PCL181, PLZ05, Type 66. Very difficult to notice 130mm still balance. Minimum two PCL181 Battalions each with one more battalion of the old equipment.
B) Minimum two PHL03 Battalions which has become three MLRS Battalions wherever PCL191 has been inducted. The induction of second PCL191 battalion in the Brigade will surely strengthen the Multi-Domain Precision Warfare capabilities which is the focus of PLA.