It's basically a full day to get from Flores to Semuc Champey on a shuttle and a full day from there to GC. Frankly, I'd leave that part out and spend the extra time at the lake. There aren't night buses so you lose 2 of your precious days sitting on your backsides in order to have just 1 full day there.
I think 3 nights is fine in Antigua - lots of colonial ruins, the parque central, neighboring villages to explore your 1 full day there. Then I'd spend at least 4 nights on the lake and include Tikal and maybe Yaxhá before you head back to GC and your flight home.
