Coding to centre blog gallery images on mobile view

Site URL: https://www.dearromance.com.au/blog/real-wedding-gareth-sarah-perrys-paddock

Is there coding I can add so the 3 picture galleries on my blog in mobile view are centred? Or have all 3 pictures in one line? They currently show 2 on one line, and the other below but it's not centred.

I'd prefer to have centred but either fix works. Please help!! Thank you

Add to Design > Custom CSS

/* Mobile-Center gallery */
@media screen and (max-width:640px) {
div#block-yui_3_17_2_1_1621498121440_26545 .slide:nth-child(3) {
    margin-left: 25%;
}
}