There is a bug in Minecraft Beta 1.16.0.55 where Fortune 3 only yields one coal and 4-5 redstone when mined. I have a diamond pickaxe with Fortune 3, Mending 1, Efficiency 4, and Unbreaking 3 and I mined 64 coal ore and got 64 coal in return. Began testing with other ores like redstone and received 4-5 everytime for 20 blocks.

Bug is in Beta 1.16.0.55! Would not let me send since 1.16 is not officially released.