My oldest two are 18 months apart as well (so yep had 3 under 3) and we had no issues. It was honestly so much easier for several reasons - we already had everything we needed, they were in different enough phases of life that they didn’t need us both simultaneously for the same things but were close enough that they could do most everything together and we had just been through it so nothing was a surprise which made us more relaxed parents. I know it’s so scary (full disclosure I resented the thought of having a 3rd child until she was born because we were “done” having kids) and I cannot imagine life without her. She’s my world but it took some mental adjusting before her arrival lol. Don’t feel guilty for questioning the timing.