I'm 63 and have been taking a Chlorthalidone 12.5mg + Metoprolol Succinate 23.75mg + Telmisartan 40mg Tablet ER for a month for blood pressure that was 160/100 - how effective is this likely to be, and would I need any additional medicine?

It's encouraging that you're tracking this closely, since a starting reading of 160/100 does need proper management. This combination works through three different mechanisms, which often makes it quite effective for readings in that range, but how well it's working for you specifically can only be judged by rechecking your blood pressure regularly and watching the trend over these weeks. Whether anything additional is needed depends on those follow-up readings, so please stay in touch with your doctor rather than adjusting anything yourself.