can't play the same music at the same time in my Era 300 and sonos 1 (gen1) but can play different songs

Why not? 
 

What have you tried? The “group” feature should work to give what you want. 

I have try everything 

if I start music in eg stereo pair play 1. press everywhere then era 300, era 300 sonos 3 i. press apply. still only music in the stereo pair. it makes different music play in all speakers

Everything?
 

Voice control? Physical buttons? Just grouping two rooms? Choosing “Everywhere” and not pressing anything else except “Apply”?
 

https://support.sonos.com/en-us/article/group-and-ungroup-rooms?language=en_US

What is a Sonos 1 (gen 1) ? Sonos has made the PLAY:1 (only one generation, runs either S1 or S2) as well as the Sonos One (two generations, both compatible with S1 or S2) and the Sonos One SL (one generation, only runs S2). 

It would help us to know both which specific speakers you’re using, and the version of the Sonos operating system you’re using. Of course, devices running S1 can not be grouped with devices running S2. 
 

Vad är en Sonos 1 (generation 1)? Sonos har tillverkat PLAY:1 (endast en generation, kör antingen S1 eller S2) samt Sonos One (två generationer, båda kompatibla med S1 eller S2) och Sonos One SL (en generation, kör endast S2).

Det skulle hjälpa oss att veta både vilka specifika högtalare du använder och vilken version av Sonos-operativsystemet du använder. Naturligtvis kan enheter som kör S1 inte grupperas med enheter som kör S2.

Sorry

Play 1 (stereo )

play 3

era 300 

Do you still use a Bridge for your older speakers? If so, remove it and connect all to your WIFI.

​@jonastjernberg You should not be pressing anything after but “apply” after you’ve crossing “Everywhere”.

Era 300 Stereo (vardagsrum) 

play 1 stereo (kök)

play 3 (Lovisa) 

only sound from one Era 300

Era 300 Stereo (vardagsrum) 

play 1 stereo (kök)

play 3 (Lovisa) 

only sound from one Era 300

What happens if you choose a different source after you’ve grouped the rooms? 

Same just sound from one Era 300