You can change which slide should load first with a script, which should load before the slider finished loading. In this example 265 is the slider's id, and 2 is the slide index (which is the index of the third slide in your slide list.)
<script> window['ss265'] = 2; </script>
You can put this code for example just before your website's </head> part.
<!DOCTYPE html> <html> <head> ... <script> window['ss265'] = 2; </script> </head>
and you could ask down for example based on the url, which slide do you want to show first.
Of course if you already know the number, and you just want to write it manually somewhere, you could put it into your page or post editor too, just switch your editor to see the html code, and write this script right before the shortcode:
<script> window['ss265'] = 2; </script> [smartslider3 slider="265"]
<script> window['ss265'] = <?php echo $slideID; ?>; </script>