new to mcreator and can't figure out random spawning

so I'm trying to make a boss that randomly summons mobs, I'm trying to make it so it has a 10% to spawn 1 of three mobs every second, problem is, i have no clue how to do this, I'm fairly new to mcreator so i don't know how many functions work. I tried making a procedure under "On Entity Tick Update" and using the procedure template "Do with a 70% chance" and modified it but it just spawn's entities every millisecond it feels like, so I'm looking for help to try and make it so 1 of 3 mob's have a 10% chance to spawn every 1 second