Annual Multi-Trip products come in varying shapes and sizes. Things to look for in your circumstances are the maximum single trip limt (many are limited to around 31 days but you should be able to find one that covers for around 100 days). Some policies may restrict the total length of time that you can spend outside the UK throughout the 12 month period.
If you know that you will be going to Worldwide destinations during the course of the year then it would probably be best to buy a policy valid for all destinations now as upgrading from Europe only during the course of the year could cost more.
In respect of the camera you will not need to cover this seperately unless the value exceeds the Single Article Limit on the policy. Generally the policy limit will be between £150 to £300 but can go higher. If this is not sufficient cover then the company providing insurance may be able to increase the limit for this item. Alternatively if you reside in a property with Home Contents insurance then you may already be covered for this item when you take it away. However, if they do provide cover then you 3 month trip may be more than the agreed duration on that policy but could be worth checking.
Cannot recommend any specific company but if you do not find an appropriate company here on MSM then check the search engines for 'Annual Multi-Trip Insurance' you should find something suitable there. Of course an Insurance Broker might be able to help you as well.
Good luck