Count arithmetic triples in a multiset under add and remove queries

Start with an empty multiset and a fixed difference d. Each query either adds one copy of x or removes every copy of x. After each query, report how many triples of elements (counting copies separately) can be ordered as x, x + d, x + 2d. Keep a count per value and update the running total by only the triples that the changed value takes part in, instead of recounting.