What kind of citation style is it?

Question

I am writing my doctoral thesis, but I am frustrated when I try to output the reference format as my school required, it's required as follow (Only the following three types are needed in my thesis),

Journal [J]

Book [M]

Conference [C]

Other requirements:

1. Multiple authors: when the number of authors are greater than 3, keep first three and use "et al" for the rest authors.
2. Sorting: By appearance.

My question is what kind of the bibliography style the above is?

and if there is none of the existing bibliography styles fitting for it, how should I build this style from scratch?

.bib:

@article{brauner91,
  author = {Brauner, N.},
  title = {Vapour absorption into falling film},
  journal = {ASME J},
  year = {1991},
  volume = {34},
  pages = {76-82},
  number = {3}
}
@book{baehr1994,
  title={Heat and mass transfer},
  author={Baehr, H. D.},
  year={1994},
  page={221}
  publisher={Berlin:Springer-Verlag}
}
@inproceedings{leiner1989,
  year  = {1989},
  pages={68--71},
  author = {Leiner, A.},
  title = {Optical techniques for heat transfer measurements},
  booktitle = { COLLINS W. Proc Int Conf on Energy and Environment},
  publisher={New York: Academic Press}
}

Answer 1

Try the following

\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage[style=numeric,backend=biber,giveninits=true,sorting=none,maxnames=3,minnames=3]{biblatex}

\addbibresource{biblatex-examples.bib}

\DeclareNameAlias{default}{family-given}
\renewcommand*{\revsdnamepunct}{}
\renewrobustcmd*{\bibinitperiod}{}
\renewcommand*{\finalnamedelim}{\multinamedelim}

\DeclareFieldFormat*{title}{#1}
\DeclareFieldFormat*{journaltitle}{#1\isdot}
\DeclareFieldFormat*{issuetitle}{#1}
\DeclareFieldFormat*{maintitle}{#1}
\DeclareFieldFormat*{booktitle}{#1}

\newbibmacro*{title}{%
  \ifboolexpr{
    test {\iffieldundef{title}}
    and
    test {\iffieldundef{subtitle}}
  }
    {}
    {\printtext[title]{%
       \printfield[titlecase]{title}%
       \setunit{\subtitlepunct}%
       \printfield[titlecase]{subtitle}}%
     \ifentrytype{article}{\setunit{}\printtext{[A]}}{}
     \ifentrytype{book}{\setunit{}\printtext{[B]}}{}%
     \ifentrytype{inproceedings}{\setunit{}\printtext{[C]}}{}
     \newunit}%
  \printfield{titleaddon}}

\renewbibmacro*{in:}{\ifentrytype{article}{}{\printtext{//}}}

\renewbibmacro*{journal+issuetitle}{%
  \usebibmacro{journal}%
  \iffieldundef{series}
    {}
    {\newunit
     \printfield{series}}%
  \setunit{\addcomma\space}%
  \printfield{issue}%
  \setunit{\addcomma\space}%
  \usebibmacro{date}
  \setunit{\addcolon\space}%
  \usebibmacro{issue}%
  \setunit{\addcomma\space}%
  \usebibmacro{volume+number+eid}%
  \setunit{\addspace}%
  \newunit}

\DeclareFieldFormat[article,periodical]{number}{\mkbibparens{#1}}
\renewbibmacro*{volume+number+eid}{%
  \printfield{volume}%
  \printfield{number}%
  \setunit{\addcomma\space}%
  \printfield{eid}}

\DeclareFieldFormat{pages}{#1}
\DeclareFieldFormat{postnote}{#1}
\DeclareFieldFormat{multipostnote}{#1}
\renewcommand*{\bibpagespunct}{\addcolon\addspace}
\renewcommand{\postnotedelim}{%
  \iffieldpages{postnote}
    {\addcolon\space}
    {\addspace}}

\begin{document}
\cite{sigfridsson,brauner91,baehr1994,leiner1989,aksin}
\printbibliography
\end{document}

Most of this is fairly standard and can be found in Guidelines for customizing biblatex styles, biblatex: remove commas between last and first names in bibliography, Colon instead of “p.” in Biblatex authoryear-comp, Biblatex: No \postnotedelim for citations that aren't numerals, biblatex: parentheses around the volume number of an article

The odd requirements of your style were the indication of the entry type after the title as we as the "//" separator for "in", they are implemented via

\renewbibmacro*{title}{%
  \ifboolexpr{
    test {\iffieldundef{title}}
    and
    test {\iffieldundef{subtitle}}
  }
    {}
    {\printtext[title]{%
       \printfield[titlecase]{title}%
       \setunit{\subtitlepunct}%
       \printfield[titlecase]{subtitle}}%
     \ifentrytype{article}{\setunit{}\printtext{[A]}}{}
     \ifentrytype{book}{\setunit{}\printtext{[B]}}{}%
     \ifentrytype{inproceedings}{\setunit{}\printtext{[C]}}{}
     \newunit}%
  \printfield{titleaddon}}

\renewbibmacro*{in:}{\ifentrytype{article}{}{\printtext{//}}}

the latter of course follows the general pattern of Suppress “In:” biblatex.