Begin by choosing photos in sets of threes, starting with the ones you want on the bottom row.
When uploading, Under layout option choose 'none' (This allows you to move them around as they will load one on top of the other in a column and you want to click on each one and drag them into a row - you choose whether you want spaces or not.) and 'small' under size.
I work in 'compose' as this is simpler for me and check how it is going with preview. I usually find the photos are still too big, so I click on each, one at a time, and drag the corner marker to make them smaller.
Finally I publish and choose 'open in a new window' so that I can check the final result and edit if necessary. If the photos aren't small enough I find they wont sit three in a row, but send the third one down to the next line, which spoils the effect.
BTW I cooked the 3 cakes, but my husband ended up doing most of the icing, including the intricate measuring of the patern, as I was not well and also needed his mathematical skill and precision and patience. I went to bed to recover for the wedding day. I managed the day but am still not well- lots of coughing, which started yesterday.
Well I just had a go at publishing a didn't get three in a row so I need to shrink them or try two in a row. Finally it worked!!