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6:13 PM
Grid[{
{1, SpanFromLeft, "A"},
{3, SpanFromLeft, SpanFromAbove},
{4, 5, "B"},
{6, 7, 8}
}, Frame -> All]

Is this closer to what you're looking for? I couldn't figure out how to remove the frame line however between "A" and "B". I don't know why the rows have different vertical span in your implementation
But the only thing I changed is I removed the newlines and put A and B in different cells. But now they have the frame line between them since Frame->All and I don't know how to tell Frame to put Frame lines everywhere except between the cells containing A and B
 

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