I've taught 4/5 never three grades though. It is tough I agree but not a whole lot tougher then teaching a straight grade. There is always a range of abilities. One challenge can be your district if they demand that your teach set "canned" programs for each grade level independantly not taking into account ability.
I found some succes setting up rotations kind of like language centers. They are all on the same topic just different levels of understanding. This takes a little kids training to set the rules for how things will work.--you know the stand stuff like what if they finish early etc. A lot depends on how many kids are in your classroom. this will work great for about 20-25 kids. If you have more students you'll need to add a 4th group to the rotation. Groups of about 8 kids. Calling on parent volunteers during this time can be really useful too if you have them Order of rotations group 1 seat work--these are the most capable kids they are working on the lesson you taught the day before. A practice sheet usually. Group 2 --centers--- This could be compters combined with practice activities like flash cards, skill games etc. This group is the middle as far as ability goes. They go to the teacher second.---these activities are the kind that are set for the whole week so you only have to plan for them once a week. group 3 will start with teacher--these are the kids that need the most instruction and are the least knowledgable on the topic. They go from teacher to seat work then to centers. These group will need to be flexible--you'll need to assess before each topic. Which takes time but it will pay off in the long run.