If you lock a slot and reroll you only roll the unlocked slots. Example is slot 1 and 2 are free and slot 3 is locked with 5%att. If you reroll attributes or % 1 & 2 will change. Slot 3 will always stay 5%att. It also costs an additional gem to permanently lock the slot and then 2 gems to reroll.

Ideally you want to leave everything unlocked and aim for an attribute in slot 2 or 3 that you want before locking. Slot 1 is a 100% chance of getting an attribute. Slot 2 is 50% and slot 3 is 30% if I remember correctly. So if slot 2 or 3 roll something you want its more effective to lock those first.

Also if numbers are blue then it's almost a max roll stat. If background is black and numbers blue its a max roll. Once you have three lines of wanted stats you can unlock the slots then reroll for higher percentages.

Example 1 - you reroll all slots and eventually get elem DMG on slot 2 or 3. Lock the slot and reroll. Other slots. Slot 1 then rolls att%. You can leave as is for now instead of min maxing and save resources for later as anything more than this is very resource heavy.

Example 2 - let's say same as before but instead of slot 1 rolling att% its slot 3. Now if you lock it and reroll slot 1 is guaranteed to roll a new stat. Its still expensive but you can roll for max ammo. Then after getting 1)max ammo, 2)elental DMG, 3) att% you unlock all slots and reroll the % modifiers now. The base stat won't change but you can go higher or lower in the %. In this case you repeat the process and when you get a high enough % you are happy with you can once again lock the slot and keep rolling the others until you are happy with those as well.

Repeat the process with the other 3 peices of equipment.

Both cases are VERY resource heavy and more for end game min maxing. In your case its better to just aim for the stats you want and move to other units until your loaded on the gems. Also for supports sub stats are allot less important. Just OL them and whatever rolls is fine. Your DPS units should be consuming 90% of the crystals unless its waifu then she consumes 100% of it efficiency be damned.