In Phases 1 (Prepare) and 2 (Deploy) we developed an understanding of the state of the nation of research software, its developers/users, its requirements, and how software is changing the way research is conducted. Building on our experience and expert understanding, Phase 3 (Expand) will focus on the creation of sustainable and self-supporting communities of practice to empower cultural change that enables better practice to be widely adopted to: foster a culture of sharing expertise and enabling; integrate project consultancy, training and awareness raising to effect and support change; move from local and individual actions to national and community level effects. Our goal is that the UK research community be enabled to take full advantage of software and, in doing so, to support the conduct of excellent research. Our objectives are: A) Widespread adoption of research best practice: agreeing and defining best practice with reference to research software, and enabling its widespread adoption to ensure the reliability and reproducibility of modern research. Enabling development of models and blueprints for initiating, nurturing and maturing communities of practice, enhancing our status in the UK and internationally as the go-to institution for insight into research software matters and catalysing new international collaborations. B) Cutting-edge policy and guidance: collaborating with stakeholders to create and disseminate evidence-based guidance, infrastructure, policies and tools. This leads to improved reusability of research software and its associated research outputs. C) A capable research community: based on a sustainable and scalable community-led model that will push the boundaries of knowledge across domains to maintain excellence and drive innovation and career paths, to increase the recognition of research software. Supporting collaboration in the UK research society and helping it become more resilient and sustainable will achieve an increased social and cultural impact. D) An open evidence bank: identifying and generating datasets, conducting analysis to provide insight and evidence of the importance of software, people and practices. This enables costing of resources required to develop, maintain and preserve research software. To achieve these goals, we will be: 1) Raising awareness: empower and develop a cohort of ambassadors for good practice through our Fellowship; outreach to stakeholders at all levels on research software issues; to deliver adoption of best practice. 2) Seeding change: build multiple sustainable Communities of Practice (CoP): for research domains, for techniques, for stakeholder groups, for UK institutions; set up expert panels that commission topic-based programmes of workshops, policy studies and outreach; nurture and scale existing CoPs e.g. through RSE exchanges; develop tools and services to support CoPs; to guarantee the widespread adoption of research best practice. 3) Providing expertise: create regional training hubs to continue growth of provision; commission new courses; refocus open call consultancy; conduct feasibility study for an RSE brokerage; to form a capable research community. 4) Influencing policy: publish new guidance and standards; work with international collaborators to put policy into practice; conduct research that improves understanding of research software; to deliver cutting-edge policy and guidance and build an open evidence bank.