Can I use dynamically linked MinGW runtime libraries (f.e. libgcc_s_sjlj-1.dll, libgfortran-3.dll and libquadmath-0.dll) with the BSD dll library for commercial purposes?
I see several issues here.
Firstly, there is no bar on using GPL software for commercial purposes. You can sell it; you must simply honour the GPL when you do so, which means giving your customers access to source, on the GPL's terms. So I think when you say commercial you mean proprietary, ie, without giving your customers access to the source of your product under the terms of the GPL.
Secondly, you have told us (here and in your earlier question) that your code is to be dynamically-linked to a DLL which is distributed under a BSD licence, and which is in turn dynamically-linked to the three GPLv3-plus-runtime-library-exception (GPLv3+RTLE) libraries in question. Because of the exception, while you are obliged to give your customers (either on request, or simply automatically) the source of the GPLv3 code you are distributing, you are not obliged to distribute the source of your own product on the same terms.
So as I understand it: you can distribute your binary, presumably alongside the BSD DLL and the three GPLv3+RTLE DLLs. You will be obliged to fulfil source distribution (and other GPLv3) obligations in respect of any GPLv3+RTLE DLLs that you distribute, but you will not be obliged also to distribute your code under GPLv3.
Of course, IANAL/IANYL, so you should take professional legal advice before betting a company on this.