The first sentence might need another "the", before "formwork". It depends whether all the formwork he built collapsed (needs "the") or only some formwork he built collapsed (doesn't need "the".)
The second sentence leaves out a needed "the" before "course" (which is a countable noun, even though each construction has only one course). We are speaking of the course of this construction, not of any other.
The third sentence is acceptable grammatically, but is not idiomatic. We would say
- "during the construction of [X],... (count noun; this construction, not some other instance)
- "during construction,..." (non-count noun)
Lastly, we would not likely say "formwork erected by him". We would say "formwork [that] he [had] erected" or "formwork [that] he [had] built". But this is just a matter of style, not of grammar.