What kind of data visualization can show 3 aspects of the data including time (the others are the number of conversions and revenue)? For example, I thought about bubble chart where size is one aspect and Y position is another, another example is a multi-set bar chart
It depends on the tradeoffs (and the story you need to convey): You can break the charts into adjacent, separate displays, or change the chart to emphasize relative change over time.
Break into separate displays:
This tradeoff means you have precision about each figure, but need to parse 2 data points. You can try to amend this with interaction with either chart:
Try making it like a sentence:
In this case, since the y-axis' are separate, hovering on one chart gives you interactivity in the next. I've seen this done vertically as well.
It might be harder to parse the trendline, but the tradeoff is accuracy. Bubble charts can be difficult, as area is hard to parse.
Another solution: if change over time (not volume) is important, change the y-axis to reflect that:
What about dual y-axis charts?
There used to be more of an emphasis on dual y-axis visualization, but they've fallen out of favor due to perception problems:
The proportions of the two scales are often different from each other in dual axis charts. If the left axis would go down to zero, the chart would be twice as long. If the right axis would go down to zero, the chart would be almost three times as long.
Without seeing the data or what correlations you want to show it's difficult to say. Sometimes you just have to see what visualization method suits the data and the users by "trying them on".
It sounds like you might benefit from trying the data in violin plots or stream graphs?
There's a good selection to review here: http://visualizationuniverse.com/charts
I'm making some hypothesis, but I suppose revenues are positively correlated to time (the longer, the more money). The conversion rate is a side information for most of decision-makers as long as the money made is big.
The bubbles also have some sense of "area" but time*something else is a strange unit, that doesn't make much sense.
The top rectangle is short, high revenues/time and high conversion/rate
The bottom rectangle is long, smaller revenues/time and medium conversion rate