For Android, the Kobo app will work but I've never tried sending it a sideloaded ebook but have read posts saying it is possible.
For Windows, I've been using Thorium which works quite well or ADE 4.5 which works for most of the FLO ePub3 ebooks I've tried.
The code block below shows how complex looking a FLO ePub 3's can look with styles applied to every individual character.
Code:
<div class="liw c7 cE cF" style="top:320px;z-index:286">
<span class="w f1" style="width:87px"><span class="c10 c" style="width:32px"> T</span><span class="c11"><span class="c c12">H</span><span class="c cD">E </span></span></span><span class="w f1" style="width:159px"><span class="c10 c c13">M</span><span class="c11"><span class="c c14">A</span><span class="c c15">G</span><span class="c c16">I</span><span class="c c14">C</span><span class="c c15">A</span><span class="c cD">L </span></span></span><span class="w f1" style="width:144px"><span class="c10 c cC">W</span><span class="c11"><span class="c c15">O</span><span class="c c14">R</span><span class="c c14">L</span><span class="c cD">D </span></span></span><span class="w c17 f1 c11" style="width:54px"><span class="c c15">O</span><span class="c cD">F </span></span><span class="w f1" style="width:123px"><span class="c10 c c18">H</span><span class="c11"><span class="c c19">A</span><span class="c c1A">R</span><span class="c c1B">R</span><span class="c cD">Y </span></span></span><span class="w" style="width:349px"><span class="f1"><span class="c10 c c12">P</span><span class="c11"><span class="c c15">O</span><span class="c c15">T</span><span class="c c19">T</span><span class="c c1C">E</span><span class="c c1D">R </span></span><span class="c1E"><span class="c c16">.</span><span class="c c16">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c16">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c c1F">.</span><span class="c cD">. </span></span></span><span class="c20"> </span></span><span class="w c19 c21 f1 c10">6</span>
</div>