Yes, there are several 64-bit Linux for Pi3, for example Arch Linux.
The reason you want to use a 64-bit OS on ARM has nothing to do with DRAM addressing (32-bit Arm7a has address extensions that allow 1 TeraByte). The AARM64 architecture has many advantages over the AARCH32; DP full floating point in hardware, deeper SIMD / NEON vector, more registers. This is about twice as fast in dp-floatng point tests, but faster for cryptography. 64 bits probably uses a little more DRAM to execute.
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