I did not stake or cage my two indoor tomato plants and they are now about 3 feet high. Should I tie twine around them now? Can you offer any other suggestions?

Yes, tie them as soon as you can, tying a loop that doesn't slip. Keep the knot loose so it doesn't bind the stem. If you do not tie them or stake them up, they will likely break under their own weight. Indeterminate will continue to grow, so plan on retying them 3-4 times and for them to get very large. Control the size of the plant by trimming where needed. You can control the bushiness by trimming the side shoots and suckers, and control the height by trimming the main stalks. Leave some new growth for continual flowers and tomato production.