I would translate "in solidum, ou qui d'entre elles mieux le devra" as "jointly or severally", where "severally" means "separately" - but you probably know that.

I think this is an unusual way of expressing a very common concept, as shown by the fact that it gets only five Google hits.

A variation on a 'most liable' theme.

It's not one or more. It's just one company in the singular : 'le devra'. I had also been gravitating towards 'severally' but that is subsumed under in solidum: jointly and severally.

Otherwise I don't think mieux can be simply ignored and a 'split costs order' scenarion needs to be brought into the frame, even though not quite on all fours with this scenario.

Code de procédure civile fr.:

Art 696: La partie perdante est condamnée aux dépens, à moins que le juge, par décision motivée, n'en mette *la totalité ou une fraction à la charge d'une autre partie*.
Art 700 : Le juge tient compte de *l'équité ou de la situation économique* de la partie condamnée. .



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Note added at 9 hrs (2022-01-12 19:06:50 GMT)
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'split costs order' needs to be brought into the frame, ....

Asker has told us this is part of a pleading, and it is apparently in support of DDD’s case.

The expression qui d'entre elles mieux le devra refers to a subsequent determination, following after the present case, in which the relative liability of the defendants will be established.

From DDD’s point of view, it is not necessary to establish the relative liability of AAA, BBB and CCC, since DDD is seeking only that all three of them be found liable in solidum for the entire value of the condemnation. Once that collective liability has been established by the court, precisely how the penalty is shared between AAA, BBB and CCC will depend on the contractual obligations binding on those three companies – details to which DDD (and indeed the court) may not have access.

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Note added at 8 hrs (2022-01-12 18:13:15 GMT)
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@Phil and AT
In the absence of full context, let's suppose that AAA is a supplier of goods, BBB is AAA's insurance provider, and CCC is a delivery service provider. Let's further suppose, that AAA sold goods to DDD, they were damaged when delivered, and DDD is seeking compensation for all his losses. A priori, any (or all) of the three companies might bear some responsibility/liability. The way the obligation to compensate is shared between them will depend on the terms of their bilateral contracts - and all three might be required to contribute to some extent. Eg: what if the liability of BBB (the insurer) is limited to the first Eur 2000 of any claim? What if CCC's liability is limited to 100% of the declared value of the goods, likely to fall well below the total value of DDD's claim for compensation in addition to that for loss of the goods themselves (principal, intérêts, frais, dommages et intérêts et accessoires)?

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Note added at 9 hrs (2022-01-12 19:49:59 GMT)
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in solidum vs. jointly and severally

This webref sheds useful light on the terms used in the ST and in some of the answers:
https://partiels-droit.com/obligation-in-solidum/

I venture to suggest that it supports the idea that DDD is asking the court only to determine the total amount of compensation, without going into the nitty-gritty of who has to pay (AAA, BBB and/or CCC) and in what proportion.

IOW, a workaround.