3 ram slot motherboard: Socket 7 Motherboards - Retro PC Store. Amazon.com: HUANANZHI X99-TF-Q ATX Motherboard with LGA 2011-3 ... . I tried to install a ddr3 into a ddr4 slot and tried to boot it.
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This means a DDR4 RAM stick physically won't fit into a DDR5 motherboard
slot.