Thanks or your question - I am Sam and I am one of the Uk tax experts here on Just Answer.
Yes you are right you would be liable to the additional 3% stamp duty, and IF yous ell this initial home more than 18 months after moving out, only then would a capital gain consideration arise.
If more than 18 months after moving out, then the time it WAS your main home is taken into account to exempt some of the gain (plus the last 18 months of ownership) plus you get additional exemption for private lettings relief, which allow up to a maximum of £40,000 furtehr tax relief AND you also have the annual exemption allowance which stands at £11,100 this year.
So you will find the capital gain bill (which is any gain left over charged at 18% or 28% or a mix of both depending on your annual income) will be low if you sell within 5 years.
Let me know if I can assist further