acro has interfaces for defining arbitrary new properties with \DeclareAcroProperty, and new commands with \NewAcroCommand detailed in the documentation in sections 29. Properties and 30. Own acronym commands.
However, for defining multiple properties and an internal interface, I think \DeclareAcroEnding can be slightly misused to good effect.
\DeclareAcroEnding{feminine}{}{} defines *-feminine and *-feminine-form equivalents to the *-plural and *-plural-form that are all initially empty.
Where the feminine form differs by letters appended to the masculine form long-feminine=les can be set, or where it differs more substantially long-feminine-form=tridimensionelles can be used to state it in its entirety.
Then we need to add new \ac-variants to apply these properties, e.g.
\NewAcroCommand \acF {m}
{ \acrofeminine \UseAcroTemplate {first} {#1} }
\NewAcroCommand \AcF {m}
{ \acrofeminine \acroupper \UseAcroTemplate {first} {#1} }
(note I'm using F to avoid clashing with the already existing \acf-type commands that force the full appearance).
Other variants need to be explicitly defined individually but can be based on the definitions found in the source.
\documentclass[12pt,twoside,openright]{book}
\usepackage{acro}[=v3]
\DeclareAcroEnding{feminine}{}{}
\DeclareAcronym{2d}{
short = 2D,
long = bidimensionnel,
long-feminine=les,
}
\DeclareAcronym{3d}{
short = 3D,
long = tridimensionnel,
long-feminine-form=tridimensionnelles,
}
\NewAcroCommand \acF {m}
{ \acrofeminine \UseAcroTemplate {first} {#1} }
\NewAcroCommand \AcF {m}
{ \acrofeminine \acroupper \UseAcroTemplate {first} {#1} }
\begin{document}
\ac{2d} \acreset{2d} \acF{2d}
\acF{3d} \acreset{3d} \ac{3d}
\printacronyms[heading=none]
\end{document}

For the feminine plural, I think we have to declare a separate ending \DeclareAcroEnding{feminineplural}{s}{s} using the same default as for the plural ending.
Unfortunately I don't see how we can inherit the feminine endings, so this will require some duplication, even if the plural just appends an s.
\documentclass[12pt,twoside,openright]{book}
\usepackage{acro}[=v3]
\DeclareAcroEnding{feminine}{}{}
\DeclareAcroEnding{feminineplural}{s}{s}
\DeclareAcronym{2d}{
short = 2D,
long = bidimensionnel,
long-feminine=les,
long-feminineplural=less
}
\NewAcroCommand \acpF {m}
{ \acrofeminineplural \UseAcroTemplate {first} {#1} }
\begin{document}
\acp{2d} \acreset{2d} \acpF{2d}
\printacronyms[heading=none]
\end{document}

A slightly nicer interface than exploiting \DeclareAcroEnding probably could be built, but not without either a lot of boiler plate code or relying on the expl3 interface.
\acais just printing thealtform it's working as intended,altis meant as an alternative short form, so\acais more of a stand-in for\acsthan\acor\acf. You might be trying to set plural forms and so want to setlong-plural-form=leswhen declaring, but I'm not quite clear, could you give a couple more usages of the acro commands and what you want them to expand to? – Dai Bowen Mar 07 '23 at 16:34long-plural-form=les– nicolas Emoptisie Mar 07 '23 at 20:14long-plural-formprobably isn't enough. So essentially you want a masculine and feminine version of each acro command (e.g.\accan be called with\acmascor\acfem,\acshaving\acsmascand\acsfem) or does the gender only come up in specific cases/forms? – Dai Bowen Mar 07 '23 at 20:38