if incurred as a result of their run for office.

https://www.washingtonpost.com/politics/trump-close-to-choosing-outside-counsel-for-russia-investigation/2017/05/22/8709f62e-3f22-11e7-9869-bac8b446820a_story.html?utm_term=.10c9ed33673f
>>But campaign finance lawyers said Trump could probably draw funds from his reelection committee to cover legal expenses related to the Russia inquiries, including money donated this year.

“When it comes to legal expenses, the test is whether the expenses would have been incurred irrespective of the campaign,” said Daniel Petalas, a Washington campaign-finance lawyer who served as the Federal Election Commission’s acting general counsel and head of enforcement. “So if the allegation is Trump — either as candidate or officeholder — is facing legal costs as a result of those statuses, then he is entitled to use his campaign funds to defray the legal expenses.” <<

Pence can use them too:
http://www.cnn.com/2017/06/16/politics/mike-pence-campaign-attorney/index.html
Vice President Mike Pence has hired a pricey personal defense lawyer amid the mounting Russia investigation -- but how exactly is he going to pay for it?
Pence, who unlike his boss is not independently wealthy, could possibly use funds from the Trump-Pence re-election campaign or from Pence's new PAC, which is hosting a fundraiser Friday, to pay Richard Cullen, the chairman of McGuireWoods LLP.

2) Trump has never stopped running for office he already has. These ongoing PR rallies could be covered by by campaign funds.

3) Leftover campaign funds can't be used for personal use BUT they can be donated to other political campaigns OR charities... like... oh say... Ivanka's or any other Trump non-profit and those non-profits can pay a salary to those that run it.
http://www.newsobserver.com/latest-news/article158526404.html

So... just because Trump has filed for re-election doesn't mean he's actually going to RUN in 2020. It could only be so he can keep the campaign funds he has, and is still raising, handy. For him it's a win-win.