You havet to put a null terminator on each string AND you have to make room for that terminator when you allocate the arrays:
char slowo1[6];
char slowo2[8];
char slowo3[7];
for (int i=0; i<5; i++) {
slowo1[i]=EEPROM.read(0+i);
}
slowo1[5]='\0';
for (int i=0; i<7; i++) {
slowo2[i]=EEPROM.read(5+i);
}
slowo2[7]='\0';
for (int i=0; i<6; i++){
slowo3[i]=EEPROM.read(12+i);
}
slowo3[6]='\0';
Serial.print(slowo1);
Serial.print(slowo2);
Serial.print(slowo3);
Serial.print("greta");