In general, the citizen of U.S.A and Austria need not visa to enter Hong Kong and allow to stay for 90 days for activities like location scout and meetings. For a crew of 3 to film a simple online ads should not need a visa permit to conduct your filming.

However, it is advisable to check with the Immigration Department (ImmD) before coming to Hong Kong.

http://www.immd.gov.hk/eng/services/index.html

If it is required. To get a visa/entry permit, submit the completed application form and supporting documents (e.g. employment contract/letter of appointment) to the ImmD directly by post or through a local sponsor. A local sponsor, usually the employing company, will be required to vouch for the applicant. The normal processing time is four weeks upon receipt of all the required documents.

https://www.fso-createhk.gov.hk/permits_forms/en/ImmD_WorkP_Eng.pdf