Relative to the USN, the PLAN is not trying to play the world's policeman or desiring to win two wars at the same time, so having "only" 1/4 of the USN's ARG's is really not a meaningful comparison. Name me another navy in the world that has or plans to have 3 ARGs. TBH for all I know the PLAN may want to have even more than 3 ARGs in the future; we just don't know. Also, SCS or ECS contingencies are more than met by any preparation for a Taiwan contingency, unless you somehow think the PLAN is trying to plan for both or all three to occur at the same time, so there is no need to add Taiwan + ECS + SCS as if their requirements have to be summed separately into some kind of greater whole.
I'm lumping ECS/SCS/Taiwan all together -- in that they would have to have enough ships (including amphibious assault) left at home to deter or act as a fleet in being, for ECS or SCS or Taiwan, not that they would have to do all three at the same time. That would be rather ridiculous.
If you agree that the PLAN is not likely to form ARGs by the early 2020s and also that they will end up with more than 6 LPDs, then the obvious solution is to assign 6 LPDs to the 3 ARGs when they actually form and the other 2-3 (or 5-6 or whatever) to lead their own ad hoc non-permanent task forces, to include LSTs, large LCUs, Zubrs, etc., as the need arises. This size of amphibious forces is already literally unheard outside of the USN.
I'm not trying to say that China's amphibious assault fleet of the 2020s will be lacking relative to all the world's navies -- obviously three LHDs and 6 LPDs would be one of the most capable amphibious assault fleets in the world, even not including LSTs.
I'm just saying that the fleet it will be lacking relative to the USN and lacking for the Chinese Navy's own requirements, if they try to pursue a USN style ARG deployment pattern while also trying to have enough amphibious assault ships to form a fleet in being for ECS, SCS and Taiwan contingincies.
The other point is that (specifically) for a Taiwan contingency, we aren't talking about just needing 3 ARGs, or even 6 ARGs, or even 10 ARGs. We are talking about every available ship that can land troops/tanks/IFVs/trucks/cargo, including civilian ships, being employed to take a beachhead and expand from there. An ARG in the scope of a Taiwan contingency is just a minor player in a major invasion. Even 3 ARGs all tasked together would just merely constitute a flanking maneuver to complement the main landing force. For SCS contingencies, a single ARG would surely be enough to accomplish whatever the PLAN wanted in that region, however unlikely such a scenario would even occur. For an ECS contingency, somewhere in between these two scenarios. All of this speaks nothing of any need to form permanent "ARG" organizational structures that include LSTs (or any other smaller unit) on a routine basis.
Yes, I wasn't suggesting that an ARG or some LPDs would be able to complete a full Taiwan or SCS or even ECS operation, but rather exist as a component of the overall fleet in being/"home fleet" that China will have as a deterrence force and potentially an initial response force.