If the oven is replacing the cooktop, what will you cook on? Are you installing a cooktop somewhere else in the kitchen? Or, are you talking about a range, not an oven? Either way, it's not just the granite countertops that you have to think about--it's also the cabinets under them. When you remove the 36"-wide cabinets that are under the 36" cooktop to install the 30" range/oven, how will you fill in the 3" on each side of the range/oven?

Why do you want to do this? Most people want to switch the other way - they want to go from a smaller cooktop surface of 30" up to a 36" wide one. And they want a separate oven at waist height instead of having to bend.

Why not replace it with a 36 inch range? Yes granite can be cut on site. The person needs to know what they're doing or they'll just make a big mess in your kitchen.

I thought of a 36 inch range, but there are not all that many choices and they are WAY more expensive that the 30 inch. If only I could find a way to deal with the extra 6 inches of space in the granite opening. Any suggestions??

How about a pic of that run of cabs/countertop.

If you can load one onto your computer, click the Houzz Photo below and then Browse to find it in your Pictures for Windows.

What you would do is cut the counter front to back at the edges of the cutout for the cooktop. A range usually doesn't use any counter. But you'll have to check the installation instructions to make sure. If you have a deeper than 24" counter you will need some of the back of the counter. In that case you will need to remove a 6" chunk of the back segment. This is so you can move the counter and shorten it. You may be able to rearrange your cabinet to work under the shorter counter or add a segment of butcherblock at an end, maybe even a waterfall end. And consider an induction range. No excess waste heat and faster/more responsive than gas.

A stainless steel filler?

Pay for a 36" range. You won't like the granite repairs, believe me. You may like them, but the person you sell your house to won't and will put the cost of the 36" range into his offer.

Who made your current cooktop? Some mftr make ovens that go under their cook tops.

Joseph is right, paying to work around that 3" on either side is never going to look right and will probably cover the expense of a 36" range, both in current costs and in future sale concession

It ain't just the lowers to worry about. The uppers will need reconfiguring to so that nothing combustible is over the cooking zone. That means 6" wider than the range for best capture. And we haven't even talked about the differing electrical requirements, etc. Then there is the ventilation needed. And make up air.

This is NOT simple or easy to do. Or cheap. It will easily end up being 10K-15K.

Sophie - the OP wants to replace a 36" cooktop with a 30" range. Assuming there is a current hood, it already would be 6" wider than the planned replacement range.

In any case, this plan is impractical and not really workable. If she cuts the granite and puts in, say, stainless steel filler strips for the 3" of counter on either side, what is supporting those strips? They need to be supported by something if they are going to be next to the range. There's many other problems, like what will filler pieces on the lower cabinets look like?

The OP should buy a 36" range. Check out the 36" NXR range on ajmadison dot com. It is $2700 and gets good reviews on this forum.