Most people would be comfortable forward facing an almost 4 year old. If you want to continue rear-facing him, the Radian is no longer the "only" extended rear-facing option.
If you want to continue rear-facing the 3 year old, you could consider:
Graco My Size/ Headwise/ Size 4 Me (same shell, different fabric/ headwings/ padding and sold at different stores). Highest height limit rear-facing, 40 pounds rear-facing. Even the tallest children will outgrown this seat by weight, not height. It runs $170-200, but goes on sale down to $145.
Radians. R100 is good for most kids (because they hit the 1.5" to shell or 44" height limit by the time they hit the 40 pound weight limit). If you want the headwings, then the RXT or if you have a heavier child the R120.
Chicco NextFit. In between the Radians & Gracos in rear-facing height, 40 pound weight. This might be a good choice for your little one--- it's very cushy and easy to get a great recline. Runs $300, on sale as low as $225.
Clek Foonf--- unless your child is very heavy, this one doesn't make a lot of sense (rear-faces to 50 pounds/ 43"). Most kids will get a little bit more time rear-facing in this seat based on height than the Radian because it has a taller shell (even though it has the lower height limit).
If you are going to forward face the 3 year old, then a combination seat makes sense. If you like the Graco Nautilus, those are still well liked. There is the Graco Argos as well, which is the Nautilus with a no rethread harness (and costs around $200). Depending on your 3 year old's stats, you could look at the Britax Frontier, which has top slots that are 1.5-2" higher than the Nautilus (so will last longer as a higher seat) but does not convert to a backless booster.
If you shop at WalMart, another option is the Harmony Defender. It's a "3-in-1" like the Nautilus, but narrower and only $100 with a no rethread harness.
What are the stats on your 3 year old? How close is he to the top of the Radian?
You have probably already done so, but here is the 5 point test to make sure your 8 year old is safe out of a booster. Height is generally more important than weight in being done with a booster, with most pre-pubescent kids still being safer/ having a better belt fit with a booster: