foo.ts

    this.firstService.getFirstResults(filterOption).pipe(
      mergeMap(firstResult => this.secondService.getSecondResults(firstResult)),
      mergeMap(secondResult => this.thirdService.getThirdResults(secondResult))
    ).subscribe(
      thirdResult => this.processResult(thirdResult, filterOption),
      null,
      () => console.log("COMPLETE")
    );