The Association of Southeast Asian Nations (ASEAN) is a regional intergovernmental organization comprising ten Southeast Asian countries. The objective of ASEAN is to promote regional cooperation and economic integration among its member states. Now, let`s examine each country mentioned in the question to determine their ASEAN membership:

1. Brunei Darussalam: Brunei Darussalam is indeed a member of ASEAN. It became a member of ASEAN on January 7, 1984, when it gained independence from the United Kingdom. Brunei`s membership in ASEAN allows it to participate in various regional initiatives and benefit from increased regional cooperation.

2. East Timor: As of my knowledge cutoff in September 2021, East Timor is not a member of ASEAN. East Timor, officially known as the Democratic Republic of Timor-Leste, is located in Southeast Asia but has not yet attained ASEAN membership. However, it is worth noting that East Timor has expressed interest in joining ASEAN in the past, and its membership application may be considered in the future. As of now, though, East Timor is not an ASEAN member.

3. Laos: Laos is a member of ASEAN. It became a member of the organization on July 23, 1997. Laos` ASEAN membership has allowed the country to engage in regional dialogue and cooperation, strengthen economic ties, and benefit from the overall ASEAN integration process.

Based on the above analysis, the correct answer to the question is Option 3: 1 and 3 only. Brunei Darussalam (Country 1) and Laos (Country 3) are both members of ASEAN. East Timor (Country 2) is not an ASEAN member, at least as of my knowledge cutoff in September 2021.