The Cause Fear spell states the following about upcasting it (emphasis mine):

When you cast this spell using a spell slot of 2nd level or higher, you can target one additional creature for each slot level above 1st. The creatures must be within 30 feet of each other when you target them.

Other spells, such as Charm Person, have identical limitations on upcasting. If you cast such a spell at 3rd level (i.e. on up to 3 targets), can you chain the spell, so that each target is within 30 feet of another target, but two of them are not within 30 feet of each other?

Visualization (X = targets, C = caster):

             C

X <--30ft--> X <--30ft--> X  

^----------65ft-----------^

To clarify, my issue is whether every creature has to be within 30 feet of every other target, or whether it is enough if they're within 30 feet of at least one other target. Just assume that all targets are within 60 feet of the caster, that's not the issue here.

Would this satisfy the condition given by the spell?

No, all of the targets must be within 30' of each other

Casting Cause Fear with a higher level spell slot allows the caster to target more creatures. All of the targets must be within 60' of the caster and within 30' of each other.

The wording of the constraint that additional targets "must be within 30' of each other" means that no target may be further than 30' from another target. The spell Chain Lightning provides an example of a different constraint more like what your diagram depicts: "each must be within 30' of the first target."