That fund is still around?  I thought no one was taking money from it because there were too many strings attached.  This is from Wikipedia, probably not up to date:

In 1977, about 29% of taxpayers checked off the box to contribute $3 of their taxes towards the fund. The level dropped to 19% by 1992 and dropped further to only 3.6% in 2020.[14] Two reasons cited for the decline are an erroneous belief that donations increase tax liability, and a general apathy toward the political duopoly.

Re-assignment of funds
The portion of the checkoff that formerly went to pay for party conventions was diverted to pay for pediatric research in 2014, with the passage of the Gabriella Miller Kids First Research Act.

The Gabriella Miller Kids First Research Act (H.R. 2019; 113th Congress), which was passed into law on April 3, 2014, diverts the money in the Presidential Election Campaign Fund which was earmarked for party conventions, to pay for research into pediatric cancer through the National Institutes of Health.The total funding for research would come to $126 million over 10 years. As of 2014, the national conventions got about 23% of their funding from the Presidential Election Campaign Fund.