The number of Balrogs changed as Tolkien's conception of the world evolved, and we shouldn't take material from the Lost Tales or Quenta Noldorinwa too literally: in the Lost Tales Beren was an Elf and the precursor of Sauron was a big cat, after all.
In the earliest concepts Balrogs weren't even Maiar, after all, so if we accept the numbers of them from those writings, should we also accept the contradiction of them not being Maiar?
By the time of Lord of the Rings the concept had changed, and a Balrog was now a destructive force capable of wiping out the mightiest Dwarf kingdom.
The figure of "3, or at most 7" comes from the Balrogs that we definitely know existed:
- The Balrog killed by (and who killed) Glorfindel
- Durin's Bane
A fourth, "Lungorthin, Lord of Balrogs", appears in the early Lay of the Children of Hurin, but there is some doubt over him (is it another name for Gothmog? is he just a Balrog lord?), and this work of course is more representative of the earlier concepts than the later.
There is, however, another named "Balrog" (not really a "Balrog", but the intention appears to be that we're talking about the same type of spirit, so I'll use "Balrog" as a convenient shorthand) in Tolkien's work who is rarely considered, but who survived to the end in all writings:
Arien the maiden was mightier than he, and she was chosen because she had not feared the heats of Laurelin, and was unhurt by them, being from the beginning a spirit of fire, whom Melkor had not deceived nor drawn to his service.
As always, remember that we're dealing with legends from a mythical age rather than a precise history here (Tolkien himself calls them "the mythology and legends of the Elder Days" in his foreword to LotR). Contradictory or confusing statements are to be expected.