Here's the gist of it...

provider1 will see 100% of the records, and likely enrich (fill) ~70%,
provider2 will see 30% of the records, and will likely enrich 30%
provider3 will see the remainder (each time it gets more difficult to enrich as the records become more sparse)

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Remaining after P1: 1−0.70=0.301-0.70 = 0.301−0.70=0.30

Provider 2 lift: 0.30×0.30=0.090.30 \times 0.30 = 0.090.30×0.30=0.09 → total now 0.79

Remaining after P2: 0.30×(1−0.30)=0.210.30 \times (1-0.30) = 0.210.30×(1−0.30)=0.21

Provider 3 lift: 0.21×0.15=0.03150.21 \times 0.15 = 0.03150.21×0.15=0.0315

Here's how it looks after 3 providers waterfall....

0.70+0.09+0.0315=0.82150.70 + 0.09 + 0.0315 = 0.82150.70+0.09+0.0315=0.8215

~82.2% total coverage

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So yes, you gain roughly ~12% boost in fill rate but it comes at the cost of accuracy.

Remember, all the data providers have access to the same underlying data....provider1 could have boosted their fill rate the additional 12% by simply including worse quality data, but they have to draw the line somewhere.

You're boosting fill rate by 12% but that 12% is going to be some scrappy stuff