[Toxaris]

Why don't you specify the italic and bold version with the same font-family name and adjust the font-style and font-weight to correspond with the actual font? That way the reader should pick the right font anyway. So, like this:

@font-face {
    font-family: "minya";
    font-weight: normal;
    font-style: normal;
    src: url("../Fonts/minya-reg.ttf");
    -webkit-hyphens:none;
    -epub-hyphens:none;
    -moz-hyphens:none;
    hyphens:none;
    -moz-font-feature-settings: "hist=1, liga=1, dlig=1";
    -ms-font-feature-settings: "hist", "liga", "dlig";
    -webkit-font-feature-settings: "hist", "liga", "dlig";
    -o-font-feature-settings: "hist", "liga", "dlig";
    font-feature-settings: "hist", "liga", "dlig";
}

@font-face {
    font-family: "minya";
    font-weight: bold;
    font-style: normal;
    src: url("../Fonts/minya-bd.ttf");
    -webkit-hyphens:none;
    -epub-hyphens:none;
    -moz-hyphens:none;
    hyphens:none;
    -moz-font-feature-settings: "hist=1, liga=1, dlig=1";
    -ms-font-feature-settings: "hist", "liga", "dlig";
    -webkit-font-feature-settings: "hist", "liga", "dlig";
    -o-font-feature-settings: "hist", "liga", "dlig";
    font-feature-settings: "hist", "liga", "dlig";
}

@font-face {
    font-family: "minya";
    font-weight: normal;
    font-style: italic;
    src: url("../Fonts/minya-it.ttf");
    -webkit-hyphens:none;
    -epub-hyphens:none;
    -moz-hyphens:none;
    hyphens:none;
    -moz-font-feature-settings: "hist=1, liga=1, dlig=1";
    -ms-font-feature-settings: "hist", "liga", "dlig";
    -webkit-font-feature-settings: "hist", "liga", "dlig";
    -o-font-feature-settings: "hist", "liga", "dlig";
    font-feature-settings: "hist", "liga", "dlig";
}

You would also not need special settings for the bold and italic tags then.

Also, be aware that some readers will ignore font-family set in the body. Also, in your snippet you forgot a closing }...