I want to buy an arcteryx shell (I'm thinking the theta sv) and I want to get a fleece to go under it so I can wear it like a 3 in one jacket. Since arcteryx does not make a 3 in one would this be a good choice to achieve the 3 in 1 effect?

The R3 might be a bit warm. 2 suggestions - 1) if you're on the move, you won't need something under the SV besides a nice baselayer (wool or capilene). Other than than, any 200-weight fleece would do just fine in my opinion. I typically use the hyllus with a wool baselayer and a OR light shell. Only ever use the 3 together when we've stopped and its windy and cold (sub 30). Around the city I find I can just toss the hyllus over a flannel and be fine to right about 35. Still works fine below 35, but if it's windy you'll feel a decent chill

Thanks Brandon, any suggestions for something to wear under the theta sv for when it's not so cold out say when it's between 35 and 50 degrees? I already own a patagonia R3 High loft fleece jacket maybe that would be best?

An excellent choice. This combo will give you the ability to have just the right weather protection across a huge range of conditions, especially on the wet/cold/windy side of the spectrum. The Theta SV and Hyllus are really durable, both in concert and solo. They were designed to work together like a 3 in 1 but with less weight, and better fit.