Hi and welcome.
Firstly I would just say - as olive trees are best suited to hot arid desert like climates with an extremely long dry season - meaning around 6 - 8 months or more without rain - they are best kept on the very, very dry side - especially as moisture is an olive tree's worst enemy - as not only does too much water quickly lead to various fungal diseases and root rot - but also a rapid untimely demise.
So as someone who grows a vast amount of olives - my immediate thoughts are - that although one or two yellow leaves are generally nothing to worry about - the amount on your trees are very definitely a sure sign that you need to cut back on the watering - particularly as every few days is way, way too much for an olive tree.
Regarding fruiting - it very much depends on the cultivar and although there are a couple that do begin fruiting as early as 3 years old - the majority don't generally start to bear fruit until the trees are somewhere between 5 to 12 years old - that said - as you mentioned that you trees haven't yet been through a winter yet - if by that you mean they were only planted this year - its unlikely that they will bear fruit until they are fully established - which could be another few years yet.
A couple of other things worth bearing in mind are that - too much shade and especially on the trunk and the lower part of tree can also inhibit fruit set - as can too much fertilizer and especially so if the nitrogen content is high - as well as the fact that - although the majority of olive trees are self pollinating - there are also quite a few cultivars that are not and of course - there many non-fruiting cultivars too - grown purely for their ornamental value.